All Powertrain Fuel & Air DTC Codes — Complete OBD-II Diagnostic Guide (2026)

This comprehensive reference covers every Powertrain Fuel & Air diagnostic trouble code (DTC) — 37 codes total. Each code includes what triggers it, common symptoms, step-by-step diagnostic procedures, and repair guidance.

37DTC Codes

37Medium

Quick Reference — Powertrain Fuel & Air Codes

DTC Code	Meaning	Severity	Common Makes
P0030	The engine control module has detected a problem with the heater circuit for the upstream oxygen sensor on bank 1. Either the sensor’s heater isn’t getting power, or the control signal is out of range, so the sensor can’t warm up quickly.	medium	Toyota, Honda, Ford
P0056	The engine control module has detected a problem with the heater circuit for the downstream oxygen sensor on bank 2, sensor 2. The heater isn’t getting the proper voltage or ground, so the sensor stays cold and can’t accurately measure exhaust oxygen.	medium	Toyota, Honda, Ford
P0100	The engine control module is telling you that the signal from the Mass Air Flow sensor is out of the expected range. This usually means the sensor, its wiring, or the connector is faulty, or something upstream is feeding the sensor bad air.	medium	Toyota, Honda, Ford
P0101	The P0101 code indicates that the engine control module (ECM) has detected an abnormal signal from the mass air flow (MAF) sensor. The MAF sensor measures the amount of air entering the engine and sends this information to the ECM to calculate fuel injection. A faulty MAF sensor or circuit issue can cause this code.	medium	Toyota, Honda, Ford
P0102	The engine computer sees a voltage that is too low coming from the mass‑air‑flow sensor. This tells the PCM that the sensor isn’t reporting air flow correctly, so it can’t calculate the right amount of fuel.	medium	Toyota, Honda, Ford
P0103	The engine control module is seeing a voltage signal from the mass‑air‑flow sensor that is higher than the calibrated maximum. This usually means the sensor, its wiring, or the PCM is reporting an unrealistic air‑flow reading.	medium	Toyota, Honda, Ford
P0106	The engine control module (ECM) is seeing a voltage signal from the MAP (Manifold Absolute Pressure) sensor that is outside its normal operating range. This usually means the sensor, its wiring, or the vacuum source is faulty, causing the ECM to receive incorrect pressure data.	medium	Toyota, Honda, Ford
P0107	The engine control module is seeing a voltage that is lower than the minimum it expects from the MAP (Manifold Absolute Pressure) sensor. This usually means the sensor, its wiring, or the sensor’s reference voltage is compromised, causing the PCM to receive inaccurate pressure data.	medium	Toyota, Honda, Ford
P0110	The engine control module is telling you that the signal from the intake‑air‑temperature sensor is out of the normal range or missing. This sensor tells the computer how cold or hot the incoming air is, which the PCM uses to set fuel and spark timing.	medium	Toyota, Honda, Ford
P0113	The engine control module sees a voltage that is higher than the calibrated range from the intake‑air‑temperature sensor. In plain English, the computer thinks the incoming air is hotter than it really is, so it leans out the mixture and may trigger a check‑engine light.	medium	Toyota, Honda, Ford
P0120	The engine control module (ECM) isn’t receiving a valid signal from the throttle‑position sensor (TPS) on circuit A. This usually means the sensor, its wiring, or the ECM input is faulty, causing the computer to think the throttle is stuck or out of range.	medium	Toyota, Honda, Ford
P0121	The engine control module is telling you that the signal from the throttle‑position sensor (TPS) on bank A is outside the expected range or fluctuates too much. In plain English, the computer cannot trust the sensor to tell it how far the throttle is open, so it limits power to protect the engine.	medium	Toyota, Honda, Ford
P0122	The engine control module (ECM) sees a voltage that is too low from the Throttle Position Sensor (TPS) “A” circuit. This usually means the sensor, its wiring, or the connector is not providing a proper signal, so the ECU can’t accurately read throttle position.	medium	Toyota, Honda
P0125	The engine control module (ECM) is telling you that the coolant temperature is lower than expected while the engine is in closed‑loop fuel mode. This usually means the coolant temperature sensor (CTS) is reading too low or the engine hasn’t warmed up enough for the ECM to switch to closed‑loop fuel control.	medium	Toyota, Honda, Ford
P0128	The P0128 code indicates that the engine control module (ECM) has detected that the coolant thermostat is not regulating temperature correctly. This can cause poor fuel economy, reduced engine performance, and potentially lead to engine damage if left unchecked. The thermostat may be stuck open or closed, or there could be issues with the cooling system.	medium	Toyota, Honda, Ford
P0130	The engine control module is telling you that the upstream oxygen sensor on bank 1 is sending a signal outside its normal range. This usually means the sensor’s wiring, the sensor itself, or its heater circuit is faulty.	medium	Toyota, Honda, Ford
P0131	The engine control module sees a voltage that is too low from the upstream oxygen sensor on bank 1. This usually means the sensor isn’t reporting enough richness, so the computer leans out the mixture.	medium	Toyota, Honda, Ford
P0133	The P0133 code indicates that the engine control module (ECM) has detected a slow response from the oxygen sensor circuit in bank 1, sensor 1. This sensor measures the oxygen levels in the exhaust gases and sends a voltage signal to the ECM. A slow response can cause the engine to run rich or lean, leading to decreased performance and fuel efficiency.	medium	Toyota, Honda, Ford
P0135	The P0135 code indicates a problem with the oxygen sensor heater circuit in bank 1, sensor 1. This sensor helps regulate fuel mixture and monitor emissions. A malfunctioning heater circuit can cause poor engine performance and decreased fuel efficiency.	medium	Honda, Toyota, Ford
P0136	The P0136 code indicates a problem with the oxygen sensor circuit in Bank 1, Sensor 2. This sensor measures the oxygen levels in the exhaust gases and sends a voltage signal to the engine control module. A malfunction in this circuit can cause poor fuel efficiency, reduced engine performance, and increased emissions.	medium	Toyota, Honda, Ford
P0137	The ECM sees a voltage that’s too low from the downstream oxygen sensor on bank 1. This usually means the sensor isn’t reporting enough oxygen, suggesting a lean condition or a wiring problem.	medium	Toyota, Honda, Ford
P0138	The engine control module is seeing a voltage that is too high from the downstream oxygen sensor on bank 1. In plain English, the sensor is reporting a constantly rich mixture even when the engine is running at idle.	medium	Toyota, Honda, Ford
P0141	The engine control module (ECM) tried to power the heater inside the downstream oxygen sensor on bank 1, sensor 2, but it didn’t get the expected response. This usually means a bad sensor, a broken wire, or a failed ECM driver.	medium	Toyota, Honda, Ford
P0155	The P0155 code indicates a problem with the oxygen sensor heater circuit in bank 2, sensor 1. This sensor helps the engine computer monitor the oxygen levels in the exhaust gases. A malfunctioning heater circuit can cause the sensor to not work properly, leading to decreased engine performance and fuel efficiency.	medium	Toyota, Honda, Ford
P0157	The engine control module detects a voltage that is too low from the downstream oxygen sensor on bank 2, sensor 2. This usually points to a wiring problem, a bad sensor, or a short to ground that prevents the sensor from sending a proper signal.	medium	Toyota, Honda, Ford
P0158	The engine control module sees an unusually high voltage from the downstream oxygen sensor on bank 2 (the sensor after the catalytic converter). It thinks the sensor is stuck rich or the wiring is shorted, so it flags a fault.	medium	Toyota, Honda, Ford
P0161	The engine control module detects a problem in the heater circuit for the downstream oxygen sensor on bank 2, sensor 2. The sensor’s heater isn’t receiving the correct voltage, so the sensor stays cold and can’t provide accurate oxygen data.	medium	Toyota, Honda, Ford
P0171	The P0171 code indicates that the engine control module (ECM) has detected a lean air-fuel mixture on bank 1, meaning there's more air than fuel in the combustion chamber. This can cause poor engine performance, decreased fuel efficiency, and potentially lead to engine damage if left unchecked.	medium	Toyota, Honda, Ford
P0172	The P0172 code indicates that the engine control module (ECM) has detected a rich fuel mixture in Bank 1, meaning there's too much fuel and not enough air. This can cause poor fuel economy, reduced performance, and increased emissions.	medium	Toyota, Honda, Ford
P0174	The P0174 code indicates that the engine control module (ECM) has detected a lean air-fuel mixture on Bank 2 of the engine. This means there's too much air and not enough fuel being injected into the engine's cylinders, causing the ECM to adjust the fuel mixture.	medium	Toyota, Honda, Ford
P0175	The P0175 code indicates that your engine control module (ECM) has detected a rich fuel mixture in Bank 2, meaning there's too much fuel and not enough air. This imbalance can cause poor engine performance, decreased fuel efficiency, and potentially damage your catalytic converter if left unchecked.	medium	Toyota, Honda, Ford
P1101	The engine control module has detected that the intake airflow system is not delivering the expected airflow. This usually means the MAF sensor signal is out of range, indicating a restriction, leak, or sensor fault.	medium	Toyota, Honda, Ford
P1135	The P1135 code indicates a problem with the heater circuit of the air/fuel ratio sensor on bank 1. This sensor helps the engine computer adjust the air/fuel mixture for optimal performance and emissions. A faulty heater circuit can cause poor engine performance, decreased fuel efficiency, and increased emissions.	medium	Toyota, Honda, Ford
P2015	The P2015 code indicates that the engine control module (ECM) has detected an issue with the intake manifold runner position sensor. This sensor helps regulate airflow to the engine, and a problem with it can cause poor engine performance and decreased fuel efficiency. The ECM uses data from this sensor to adjust the air-fuel mixture, and if the sensor is faulty, it can lead to decreased engine performance and increased emissions.	medium	Toyota, Honda, Ford
SYMPTOM-BAD-GAS-MILEAGE	The SYMPTOM-BAD-GAS-MILEAGE code indicates that your vehicle's fuel economy has decreased. This can be caused by issues with the fuel system, air intake, or engine performance. The powertrain control module (PCM) monitors various engine parameters to optimize fuel efficiency.	medium	Toyota, Honda, Ford
SYMPTOM-BLACK-SMOKE	The engine is delivering more fuel than air, creating a fuel‑rich mixture that burns incompletely. The excess fuel exits the exhaust as thick black smoke, indicating a problem with fuel delivery, airflow measurement, or combustion control.	medium	Toyota, Honda, Ford
SYMPTOM-HARD-START	The SYMPTOM-HARD-START code tells the powertrain control module that the engine needs more than normal cranking effort to start. It usually appears when the fuel‑air mixture, ignition timing, or starter torque is out of spec, causing the engine to turn over several times before it fires.	medium	Toyota, Honda, Ford

Detailed DTC Code Guides

P0030: The engine control module has detected a problem with the heater circuit for the upstream oxygen sensor on bank 1. Either the sensor’s heater isn’t getting power, or the control signal is out of range, so the sensor can’t warm up quickly.

medium ⏱️ 30-60 minutes ⚠️ You can keep the car running, but it’s unsafe to ignore for long.

Fix OBD-II code P0030: HO2S Heater Control Circuit (Bank 1, Sensor 1). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of wiring and connectors
Check for stored freeze‑frame data
Measure heater circuit voltage
...see all 6 steps →

Read full diagnostic guide →

P0056: The engine control module has detected a problem with the heater circuit for the downstream oxygen sensor on bank 2, sensor 2. The heater isn’t getting the proper voltage or ground, so the sensor stays cold and can’t accurately measure exhaust oxygen.

medium ⏱️ 30-60 minutes ⚠️ You can keep driving short distances, but the code will likely cause a failed emissions test.

Fix OBD-II code P0056: HO2S Heater Control Circuit (Bank 2, Sensor 2). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of wiring and connectors
Scan tool live data check
Heater circuit voltage test
...see all 5 steps →

Read full diagnostic guide →

P0100: The engine control module is telling you that the signal from the Mass Air Flow sensor is out of the expected range. This usually means the sensor, its wiring, or the connector is faulty, or something upstream is feeding the sensor bad air.

medium ⏱️ 30-60 minutes ⚠️ You can keep driving, but expect reduced fuel economy and possible emissions‑test failure.

Fix OBD-II code P0100: Mass Air Flow (MAF) Circuit Malfunction. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of MAF sensor and wiring
Scan live data and compare MAF vs RPM
Check for vacuum or intake leaks
...see all 6 steps →

Read full diagnostic guide →

P0101: The P0101 code indicates that the engine control module (ECM) has detected an abnormal signal from the mass air flow (MAF) sensor. The MAF sensor measures the amount of air entering the engine and sends this information to the ECM to calculate fuel injection. A faulty MAF sensor or circuit issue can cause this code.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but it may cause reduced engine performance and decreased fuel efficiency.

Fix OBD-II code P0101 with our step-by-step guide. Learn causes, cost, and DIY fixes for Mass Air Flow (MAF) Circuit Range/Performance issues. Free repair...

Diagnostic steps:

Check for obvious issues
Scan for additional codes
Monitor live data
...see all 5 steps →

Read full diagnostic guide →

P0102: The engine computer sees a voltage that is too low coming from the mass‑air‑flow sensor. This tells the PCM that the sensor isn’t reporting air flow correctly, so it can’t calculate the right amount of fuel.

medium ⏱️ 30‑60 minutes ⚠️ Yes, you can keep driving, but expect rough idle, poor fuel economy and possible emissions‑test failure.

Fix OBD‑II code P0102: Mass Air Flow (MAF) Circuit Low Input. Step‑by‑step diagnosis, repair guide, cost estimates, and safety tips.

Diagnostic steps:

Inspect Wiring and Connectors
Check for Vacuum Leaks
Read Live MAF Data
...see all 5 steps →

Read full diagnostic guide →

P0103: The engine control module is seeing a voltage signal from the mass‑air‑flow sensor that is higher than the calibrated maximum. This usually means the sensor, its wiring, or the PCM is reporting an unrealistic air‑flow reading.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can keep driving, but expect reduced power, poorer fuel economy, and a possible emissions‑test failure.

Fix OBD-II code P0103: Mass Air Flow (MAF) Circuit High Input. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Check the connector and wiring
Clear the code and watch for return
Examine the air filter and intake for leaks
...see all 7 steps →

Read full diagnostic guide →

P0106: The engine control module (ECM) is seeing a voltage signal from the MAP (Manifold Absolute Pressure) sensor that is outside its normal operating range. This usually means the sensor, its wiring, or the vacuum source is faulty, causing the ECM to receive incorrect pressure data.

medium ⏱️ 30‑60 minutes ⚠️ Yes, you can drive short distances, but avoid high load until repaired.

Fix OBD‑II code P0106: MAP/Barometric Pressure Circuit Range/Performance. Step‑by‑step diagnosis, repair costs, and emissions impact.

Diagnostic steps:

Visual inspection of sensor and connector
Check for vacuum leaks
Measure MAP sensor voltage with a digital multimeter
...see all 6 steps →

Read full diagnostic guide →

P0107: The engine control module is seeing a voltage that is lower than the minimum it expects from the MAP (Manifold Absolute Pressure) sensor. This usually means the sensor, its wiring, or the sensor’s reference voltage is compromised, causing the PCM to receive inaccurate pressure data.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can keep driving, but expect rough idle and reduced fuel economy; avoid long trips until it’s fixed.

Fix OBD-II code P0107: MAP/Barometric Pressure Circuit Low Input. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual Inspection – Loose Cap, Connector, and Wiring
Live Data Scan – Check MAP Voltage
Multimeter Test – Sensor Ground and Reference Voltage
...see all 7 steps →

Read full diagnostic guide →

P0110: The engine control module is telling you that the signal from the intake‑air‑temperature sensor is out of the normal range or missing. This sensor tells the computer how cold or hot the incoming air is, which the PCM uses to set fuel and spark timing.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can usually drive, but intermittent IAT faults may cause rough idle or reduced fuel economy.

Fix OBD-II code P0110: Intake Air Temperature (IAT) Circuit Malfunction. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual Inspection – Loose cap or connector
Scan Tool Live‑Data Check
Voltage Test at the IAT Sensor
...see all 5 steps →

Read full diagnostic guide →

P0113: The engine control module sees a voltage that is higher than the calibrated range from the intake‑air‑temperature sensor. In plain English, the computer thinks the incoming air is hotter than it really is, so it leans out the mixture and may trigger a check‑engine light.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can keep driving, but the engine may run rough, fuel economy drops, and you could fail an emissions test.

Fix OBD-II code P0113: Intake Air Temperature (IAT) Circuit High Input. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of sensor, connector, and wiring
Read live data with an OBD‑II scanner
Test the IAT sensor voltage with a digital multimeter
...see all 6 steps →

Read full diagnostic guide →

P0120: The engine control module (ECM) isn’t receiving a valid signal from the throttle‑position sensor (TPS) on circuit A. This usually means the sensor, its wiring, or the ECM input is faulty, causing the computer to think the throttle is stuck or out of range.

medium ⏱️ 30-60 minutes ⚠️ You can keep the car moving, but expect rough idle and reduced power.

Fix OBD-II code P0120: Throttle Position Sensor/Switch A Circuit Malfunction. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of wiring and connector
Check for stored trouble codes and live data
Measure TPS voltage at the sensor
...see all 7 steps →

Read full diagnostic guide →

P0121: The engine control module is telling you that the signal from the throttle‑position sensor (TPS) on bank A is outside the expected range or fluctuates too much. In plain English, the computer cannot trust the sensor to tell it how far the throttle is open, so it limits power to protect the engine.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can drive but expect reduced throttle response and possible stalling.

Fix OBD-II code P0121: Throttle Position Sensor/Switch A Range/Performance. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of the TPS and wiring
Read live TPS data with an OBD‑II scanner
Voltage test of the TPS sensor
...see all 6 steps →

Read full diagnostic guide →

P0122: The engine control module (ECM) sees a voltage that is too low from the Throttle Position Sensor (TPS) “A” circuit. This usually means the sensor, its wiring, or the connector is not providing a proper signal, so the ECU can’t accurately read throttle position.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can drive short distances but avoid high load until fixed.

Fix OBD-II code P0122: Throttle Position Sensor/Switch A Circuit Low. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of sensor and wiring
Check battery voltage and ground integrity
Measure TPS voltage at idle
...see all 6 steps →

Read full diagnostic guide →

P0125: The engine control module (ECM) is telling you that the coolant temperature is lower than expected while the engine is in closed‑loop fuel mode. This usually means the coolant temperature sensor (CTS) is reading too low or the engine hasn’t warmed up enough for the ECM to switch to closed‑loop fuel control.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can still drive, but the engine may run rich and fuel economy will drop.

Fix OBD-II code P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Check the gas cap
Verify the coolant temperature sensor reading
Inspect the sensor and wiring
...see all 8 steps →

Read full diagnostic guide →

P0128: The P0128 code indicates that the engine control module (ECM) has detected that the coolant thermostat is not regulating temperature correctly. This can cause poor fuel economy, reduced engine performance, and potentially lead to engine damage if left unchecked. The thermostat may be stuck open or closed, or there could be issues with the cooling system.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but it's essential to address it promptly to avoid potential engine damage.

Learn to diagnose and fix the P0128 code, which indicates the coolant thermostat is below regulating temperature. Get a cost estimate, common causes, and...

Diagnostic steps:

Verify Coolant Level and Temperature
Inspect the Thermostat
Check the Coolant Temperature Sensor
...see all 5 steps →

Read full diagnostic guide →

P0130: The engine control module is telling you that the upstream oxygen sensor on bank 1 is sending a signal outside its normal range. This usually means the sensor’s wiring, the sensor itself, or its heater circuit is faulty.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can drive, but expect poorer fuel economy and a likely emissions‑test failure.

Fix OBD-II code P0130: O2 Sensor Circuit Malfunction (Bank 1, Sensor 1). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection – connector, hose, gas cap
Scan live O2‑sensor data
Measure sensor voltage with a multimeter
...see all 5 steps →

Read full diagnostic guide →

P0131: The engine control module sees a voltage that is too low from the upstream oxygen sensor on bank 1. This usually means the sensor isn’t reporting enough richness, so the computer leans out the mixture.

medium ⏱️ 30-60 minutes ⚠️ You can keep driving, but watch for rough idle and poor fuel economy.

Fix OBD-II code P0131: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection – gas cap and sensor connector
Live data scan – O₂ sensor voltage
Wiring continuity and resistance test
...see all 6 steps →

Read full diagnostic guide →

P0133: The P0133 code indicates that the engine control module (ECM) has detected a slow response from the oxygen sensor circuit in bank 1, sensor 1. This sensor measures the oxygen levels in the exhaust gases and sends a voltage signal to the ECM. A slow response can cause the engine to run rich or lean, leading to decreased performance and fuel efficiency.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but it may cause reduced engine performance and decreased fuel efficiency. However, prolonged driving with a faulty oxygen sensor can lead to costly repairs, such as damage to the catalytic converter.

Learn to diagnose and fix OBD-II code P0133, which indicates a slow response from the oxygen sensor circuit in bank 1, sensor 1. Get a cost estimate and DIY...

Diagnostic steps:

Preliminary Checks
Scan for Other Codes
Check Oxygen Sensor Voltage
...see all 6 steps →

Read full diagnostic guide →

P0135: The P0135 code indicates a problem with the oxygen sensor heater circuit in bank 1, sensor 1. This sensor helps regulate fuel mixture and monitor emissions. A malfunctioning heater circuit can cause poor engine performance and decreased fuel efficiency.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with a P0135 code, but monitor your fuel efficiency and engine performance.

Fix OBD-II code P0135 with our step-by-step guide. Covers honda civic p0135 fix. Free DIY repair guide with tools, parts & cost estimates.

Diagnostic steps:

Check the basics
Scan for additional codes
Inspect the oxygen sensor and wiring
...see all 5 steps →

Read full diagnostic guide →

P0136: The P0136 code indicates a problem with the oxygen sensor circuit in Bank 1, Sensor 2. This sensor measures the oxygen levels in the exhaust gases and sends a voltage signal to the engine control module. A malfunction in this circuit can cause poor fuel efficiency, reduced engine performance, and increased emissions.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but monitor your fuel efficiency and engine performance.

Learn to diagnose and fix OBD-II code P0136, which indicates an O2 sensor circuit malfunction in Bank 1, Sensor 2. Get cost estimates, DIY steps, and common...

Diagnostic steps:

Check for Obvious Issues
Monitor Live Data
Perform Voltage Checks
...see all 4 steps →

Read full diagnostic guide →

P0137: The ECM sees a voltage that’s too low from the downstream oxygen sensor on bank 1. This usually means the sensor isn’t reporting enough oxygen, suggesting a lean condition or a wiring problem.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can drive short distances, but avoid heavy loads until fixed.

Fix OBD-II code P0137: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual Inspection – Connectors, Wiring, and Gas Cap
Read Live Data – O₂ Voltage and Fuel Trim
Voltage Test – Sensor Signal with Multimeter
...see all 6 steps →

Read full diagnostic guide →

P0138: The engine control module is seeing a voltage that is too high from the downstream oxygen sensor on bank 1. In plain English, the sensor is reporting a constantly rich mixture even when the engine is running at idle.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can keep driving, but prolonged operation may worsen fuel economy and cause an emissions‑test failure.

Fix OBD-II code P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of sensor and wiring
Read live O₂ sensor data
Signal‑wire voltage test
...see all 6 steps →

Read full diagnostic guide →

P0141: The engine control module (ECM) tried to power the heater inside the downstream oxygen sensor on bank 1, sensor 2, but it didn’t get the expected response. This usually means a bad sensor, a broken wire, or a failed ECM driver.

medium ⏱️ 30-60 minutes ⚠️ You can keep driving short distances, but the code will hurt fuel economy and may cause an emissions test failure.

Fix OBD-II code P0141: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection of connectors and wiring
Read live data and freeze‑frame info
Test heater circuit continuity
...see all 6 steps →

Read full diagnostic guide →

P0155: The P0155 code indicates a problem with the oxygen sensor heater circuit in bank 2, sensor 1. This sensor helps the engine computer monitor the oxygen levels in the exhaust gases. A malfunctioning heater circuit can cause the sensor to not work properly, leading to decreased engine performance and fuel efficiency.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but it may cause reduced engine performance and decreased fuel efficiency. However, prolonged driving with a faulty O2 sensor can lead to increased emissions and potentially damage the catalytic converter.

Learn to diagnose and fix OBD-II code P0155, which indicates an O2 sensor heater circuit malfunction in bank 2, sensor 1. Get a cost estimate and find out...

Diagnostic steps:

Preliminary Checks
Scan for Other Codes
Check the O2 Sensor Heater Circuit Voltage
...see all 5 steps →

Read full diagnostic guide →

P0157: The engine control module detects a voltage that is too low from the downstream oxygen sensor on bank 2, sensor 2. This usually points to a wiring problem, a bad sensor, or a short to ground that prevents the sensor from sending a proper signal.

medium ⏱️ 30-60 minutes ⚠️ You can keep the car moving, but expect reduced fuel economy and possible emissions failure.

Fix OBD-II code P0157: O2 Sensor Circuit Low Voltage (Bank 2, Sensor 2). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Inspect Connector and Wiring
Measure Sensor Voltage at Idle
Test Sensor Heater Circuit
...see all 5 steps →

Read full diagnostic guide →

P0158: The engine control module sees an unusually high voltage from the downstream oxygen sensor on bank 2 (the sensor after the catalytic converter). It thinks the sensor is stuck rich or the wiring is shorted, so it flags a fault.

medium ⏱️ 30-60 minutes ⚠️ You can keep driving, but watch for rough idle or loss of power.

Fix OBD-II code P0158: O2 Sensor Circuit High Voltage (Bank 2, Sensor 2). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Inspect Connector and Wiring
Check for Exhaust Leaks Upstream of Sensor
Measure Sensor Voltage at Idle
...see all 7 steps →

Read full diagnostic guide →

P0161: The engine control module detects a problem in the heater circuit for the downstream oxygen sensor on bank 2, sensor 2. The sensor’s heater isn’t receiving the correct voltage, so the sensor stays cold and can’t provide accurate oxygen data.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can drive short distances but the heater may stay off, increasing emissions and fuel use.

Fix OBD-II code P0161: O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 2). Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual Inspection of Connector and Wiring
Check Live Data with a Scan Tool
Measure Heater Resistance
...see all 5 steps →

Read full diagnostic guide →

P0171: The P0171 code indicates that the engine control module (ECM) has detected a lean air-fuel mixture on bank 1, meaning there's more air than fuel in the combustion chamber. This can cause poor engine performance, decreased fuel efficiency, and potentially lead to engine damage if left unchecked.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but it's essential to address it promptly to prevent potential damage to your catalytic converter and engine.

Learn to diagnose and fix OBD-II code P0171, indicating a system too lean condition on bank 1. Understand the causes, symptoms, and repair steps for this code.

Diagnostic steps:

Preliminary Checks
Monitoring Live Data
Inspecting the Mass Airflow Sensor (MAF)
...see all 5 steps →

Read full diagnostic guide →

P0172: The P0172 code indicates that the engine control module (ECM) has detected a rich fuel mixture in Bank 1, meaning there's too much fuel and not enough air. This can cause poor fuel economy, reduced performance, and increased emissions.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with a P0172 code, but it's essential to address the issue to prevent potential damage to your catalytic converter and reduce emissions.

Learn to diagnose and fix OBD-II code P0172, which means your engine is running too rich on Bank 1. Get cost estimates, DIY steps, and common vehicles affected.

Diagnostic steps:

Check for Obvious Issues
Scan for Additional Codes
Monitor Live Data
...see all 5 steps →

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P0174: The P0174 code indicates that the engine control module (ECM) has detected a lean air-fuel mixture on Bank 2 of the engine. This means there's too much air and not enough fuel being injected into the engine's cylinders, causing the ECM to adjust the fuel mixture.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but it's essential to address the issue to prevent potential engine damage and decreased performance.

Learn to diagnose and fix OBD-II code P0174, indicating a System Too Lean condition on Bank 2. Understand causes, symptoms, and repair costs to get back on...

Diagnostic steps:

Check for Obvious Issues
Monitor Live Data PIDs
Inspect MAF Sensor and Air Intake System
...see all 5 steps →

Read full diagnostic guide →

P0175: The P0175 code indicates that your engine control module (ECM) has detected a rich fuel mixture in Bank 2, meaning there's too much fuel and not enough air. This imbalance can cause poor engine performance, decreased fuel efficiency, and potentially damage your catalytic converter if left unchecked.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with a P0175 code, but it's essential to address the issue promptly to avoid reduced engine performance and potential damage to your catalytic converter.

Learn to diagnose and fix OBD-II code P0175 (System Too Rich, Bank 2) with our comprehensive guide. Discover causes, symptoms, and repair costs for this...

Diagnostic steps:

Check for Obvious Issues
Scan for Additional Codes
Inspect and Clean the Mass Airflow Sensor (MAF)
...see all 5 steps →

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P1101: The engine control module has detected that the intake airflow system is not delivering the expected airflow. This usually means the MAF sensor signal is out of range, indicating a restriction, leak, or sensor fault.

medium ⏱️ 30-60 minutes ⚠️ You can drive, but performance will suffer and emissions may fail.

Fix OBD-II code P1101: Intake Airflow System Performance. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Check for obvious issues
Scan live data and freeze‑frame
Test MAF sensor voltage
...see all 5 steps →

Read full diagnostic guide →

P1135: The P1135 code indicates a problem with the heater circuit of the air/fuel ratio sensor on bank 1. This sensor helps the engine computer adjust the air/fuel mixture for optimal performance and emissions. A faulty heater circuit can cause poor engine performance, decreased fuel efficiency, and increased emissions.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but monitor your engine performance and fuel efficiency closely. A faulty air/fuel ratio sensor can cause poor engine performance, decreased fuel efficiency, and increased emissions, potentially leading to costly repairs if left unchecked.

Learn to diagnose and fix OBD-II code P1135, which indicates a problem with the air/fuel ratio sensor heater circuit response on bank 1. Get cost estimates,...

Diagnostic steps:

Preliminary Checks
Scan for Additional Codes
Measure Sensor Heater Circuit Resistance
...see all 5 steps →

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P2015: The P2015 code indicates that the engine control module (ECM) has detected an issue with the intake manifold runner position sensor. This sensor helps regulate airflow to the engine, and a problem with it can cause poor engine performance and decreased fuel efficiency. The ECM uses data from this sensor to adjust the air-fuel mixture, and if the sensor is faulty, it can lead to decreased engine performance and increased emissions.

medium ⏱️ 30-60 minutes ⚠️ You can still drive with this code, but you may experience reduced engine performance and fuel efficiency.

Learn to diagnose and fix OBD-II code P2015, which indicates an issue with the intake manifold runner position sensor. Get cost estimates, DIY steps, and...

Diagnostic steps:

Check for Obvious Issues
Scan for Other Codes
Check Sensor Voltage
...see all 5 steps →

Read full diagnostic guide →

SYMPTOM-BAD-GAS-MILEAGE: The SYMPTOM-BAD-GAS-MILEAGE code indicates that your vehicle's fuel economy has decreased. This can be caused by issues with the fuel system, air intake, or engine performance. The powertrain control module (PCM) monitors various engine parameters to optimize fuel efficiency.

medium ⏱️ 30-60 minutes ⚠️ You can still drive, but monitor your fuel efficiency and engine performance. A faulty oxygen sensor or clogged air filter can lead to decreased fuel efficiency, increased emissions, and potentially cause engine damage if left unaddressed.

Fix OBD-II code SYMPTOM-BAD-GAS-MILEAGE with our step-by-step guide. Learn causes of bad gas mileage, DIY fixes, and cost estimates. Covers bad gas mileage,...

Diagnostic steps:

Check for obvious issues
Monitor live data PIDs
Inspect the air filter and intake system
...see all 4 steps →

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SYMPTOM-BLACK-SMOKE: The engine is delivering more fuel than air, creating a fuel‑rich mixture that burns incompletely. The excess fuel exits the exhaust as thick black smoke, indicating a problem with fuel delivery, airflow measurement, or combustion control.

medium ⏱️ 30-60 minutes ⚠️ Yes, you can drive short distances, but avoid high loads until the cause is fixed.

Fix OBD-II code SYMPTOM-BLACK-SMOKE: Black smoke from exhaust — running rich diagnosis. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Visual inspection & loose‑fit items
Scan live data (MAF, O₂, fuel trims)
Test the Mass Air Flow (MAF) sensor
...see all 7 steps →

Read full diagnostic guide →

SYMPTOM-HARD-START: The SYMPTOM-HARD-START code tells the powertrain control module that the engine needs more than normal cranking effort to start. It usually appears when the fuel‑air mixture, ignition timing, or starter torque is out of spec, causing the engine to turn over several times before it fires.

medium ⏱️ 30-60 minutes ⚠️ You can keep driving, but repeated hard starts increase wear on the starter and battery.

Fix OBD-II code SYMPTOM-HARD-START: Car hard to start — takes multiple tries to turn over. Step-by-step diagnosis and repair guide.

Diagnostic steps:

Check for obvious issues
Verify battery health and starter voltage
Read live data for fuel‑air sensor readings
...see all 7 steps →

Read full diagnostic guide →

Understanding Powertrain Fuel & Air Codes

Powertrain Fuel & Air diagnostic trouble codes are set by your vehicle's onboard computer (ECU/PCM) when it detects a problem in the fuel & air system. Here's what you need to know:

How codes are triggered: Sensors monitor fuel & air performance. When readings fall outside expected ranges, the ECU stores a DTC and illuminates the check engine light.
Freeze frame data: When a code is set, the ECU also records engine conditions at that moment. This data helps pinpoint the exact cause.
Pending vs. confirmed: A code becomes "confirmed" after multiple drive cycles with the same fault. Pending codes may clear on their own.

Tools You'll Need for Diagnosis

OBD-II scanner — Essential for reading and clearing codes. A basic scanner works for most codes.
Digital multimeter — For testing sensor voltages, resistance, and continuity.
Repair manual — Vehicle-specific wiring diagrams and specifications.
Safety equipment — Gloves, safety glasses, jack stands for under-vehicle work.

When to See a Mechanic

While many fuel & air codes can be diagnosed at home, see a professional if:

The code is critical severity — driving may cause further damage
Multiple related codes are present — suggests a deeper systemic issue
You don't have the tools or experience for the required diagnostic steps
The repair involves fuel system, exhaust, or catalytic converter work