All Powertrain Emissions DTC Codes — Complete OBD-II Diagnostic Guide (2026)
This comprehensive reference covers every Powertrain Emissions diagnostic trouble code (DTC) — 18 codes total. Each code includes what triggers it, common symptoms, step-by-step diagnostic procedures, and repair guidance.
Detailed DTC Code Guides
P0401: The engine control module (ECM) has detected that the Exhaust Gas Recirculation (EGR) system is not delivering enough exhaust gases back into the intake. The restriction can be caused by a stuck valve, clogged passages, a vacuum/boost‑control problem, or a wiring fault.
Fix OBD‑II code P0401: EGR Flow Insufficient Detected. Step‑by‑step diagnosis, repair guide, cost guide, and safety tips.
- Quick visual scan
- Read live data and freeze‑frame
- Test the EGR valve actuator
- ...see all 6 steps →
P0402: The engine control module (ECM) has detected that more exhaust gas is being recirculated than the system expects. This usually means the EGR valve is stuck open, a vacuum leak, or a faulty flow sensor, causing too much exhaust to re‑enter the intake.
Fix OBD‑II code P0402: EGR Flow Excessive Detected. Step‑by‑step diagnosis, repair guide, cost guide, and safety tips.
- Visual inspection of EGR components
- Scan live data for EGR position
- Test EGR valve operation
- ...see all 6 steps →
P0410: The engine control module has detected that the secondary‑air injection (SAI) system is not delivering the extra air needed during cold‑start enrichment. This leads to a richer mixture, higher emissions, and a illuminated check‑engine light.
Step‑by‑step guide to diagnosing and fixing OBD‑II code P0410. Includes safety tips, cost estimates, vehicle‑specific notes, and emissions impact.
- Visual Inspection & Quick Check
- Battery Disconnect & Reset
- Live Data Scan – SAI Valve Duty Cycle
- ...see all 6 steps →
P0420: The P0420 code means your catalytic converter is not cleaning exhaust gases as efficiently as it should. The downstream oxygen sensor detects that the catalyst is underperforming compared to the upstream sensor readings.
Fix OBD‑II code P0420: Catalyst System Efficiency Below Threshold. Step‑by‑step diagnosis and repair guide for Toyota, Honda, Ford, Chevy and other vehicles (updated for 2026).
- Check for other stored codes first
- Inspect the oxygen sensors
- Check for exhaust leaks
- ...see all 6 steps →
P0430: The engine control module has determined that the catalytic converter on bank 2 is not cleaning the exhaust as efficiently as it should. The downstream oxygen sensor reports a higher‑than‑expected oxygen level, indicating the converter isn’t doing its job.
Fix OBD‑II code P0430: Catalyst System Efficiency Below Threshold (Bank 2). Step‑by‑step diagnosis, repair options, cost guide, and safety tips.
- Visual and Quick Sensor Check
- Live Data Scan – O₂ Sensor Waveforms
- Fuel Trim Verification
- ...see all 5 steps →
P0440: The engine control module has detected a problem with the evaporative emission (EVAP) system – the network that captures fuel vapors from the fuel tank and routes them to the engine for combustion. A fault means the system isn’t sealing or venting correctly, so vapors can escape or the system can’t be purged on command.
Fix OBD-II code P0440: EVAP Emission Control System Malfunction. Step-by-step diagnosis and repair guide.
- Check the gas cap and obvious visual clues
- Scan for stored and pending codes, read freeze‑frame data
- Test the EVAP purge valve operation
- ...see all 6 steps →
P0441: The engine control module has detected that the EVAP purge valve is allowing too much fuel vapor into the intake, or not closing properly. This abnormal purge flow trips the P0441 code and may affect emissions compliance.
Fix OBD-II code P0441: EVAP Emission Control System Incorrect Purge Flow. Step-by-step diagnosis and repair guide.
- Check Gas Cap and EVAP Hoses
- Scan Live Data for Purge Valve Duty Cycle
- Test the Purge Valve Solenoid
- ...see all 5 steps →
P0442: The engine control module has found a small leak in the evaporative emissions (EVAP) system. This system captures fuel vapors from the tank and stores them in a charcoal canister. A tiny leak lets vapors escape, triggering the P0442 code.
Fix OBD-II code P0442: EVAP System Leak Detected (Small Leak). Step-by-step diagnosis and repair guide.
- Read the diagnostic trouble code
P0443: The engine control module has detected a problem with the electrical circuit that operates the EVAP purge control valve. The valve may be stuck open, stuck closed, or the wiring/connector may be shorted or open, preventing the system from properly purging fuel vapors.
Fix OBD-II code P0443: EVAP Purge Control Valve Circuit Malfunction. Step-by-step diagnosis and repair guide.
- Check the gas cap and obvious visual issues
- Read live data and freeze‑frame information
- Test the purge valve resistance
- ...see all 6 steps →
P0446: The engine control module has detected a problem with the vent control circuit of the evaporative emissions (EVAP) system. The vent valve or its wiring isn’t opening or closing when the computer commands it, so fuel vapors cannot be vented properly.
Fix OBD‑II code P0446: EVAP Vent Control Circuit Malfunction. Step‑by‑step diagnosis, repair guide, cost guide, and emissions impact.
- Check the gas cap and visual inspection
- Scan live data for vent valve command
- Test voltage at the vent valve supply
- ...see all 5 steps →
P0449: The engine control module has detected a problem with the vent valve or its control circuit in the evaporative‑emission (EVAP) system. The valve may not open/close when commanded, or the wiring may be shorted, open, or have abnormal resistance.
Fix OBD‑II code P0449: EVAP Vent Valve/Solenoid Circuit Malfunction. Step‑by‑step diagnosis, repair costs, and safety tips.
- Visual inspection of the vent valve and wiring
- Check for loose or missing gas cap
- Scan live data for vent valve command and status
- ...see all 6 steps →
P0451: The engine control module has detected that the EVAP‑system pressure sensor is reporting values outside its calibrated range. The cause is usually a faulty sensor, damaged wiring, or an over‑pressurized EVAP system.
Fix OBD‑II code P0451: EVAP Pressure Sensor Range/Performance. Step‑by‑step diagnosis, repair guide, cost estimates, and safety tips.
- Inspect the gas cap and connector
- Read live sensor voltage
- Test sensor power supply
- ...see all 5 steps →
P0452: The engine control module has detected that the pressure sensor in the evaporative emission (EVAP) system is reporting a voltage lower than the expected range. In plain English, the sensor that tells the computer how much pressure is in the fuel‑vapour tank isn’t sending a strong enough signal.
Fix OBD-II code P0452: EVAP Pressure Sensor Low Input. Step-by-step diagnosis and repair guide.
- Check the gas cap and visual inspection
- Scan live data for EVAP sensor voltage
- Test sensor resistance with a multimeter
- ...see all 5 steps →
P0455: The engine control module has sensed a large leak in the evaporative emissions (EVAP) system. The leak is big enough that the system can’t hold pressure during the self‑test, so the PCM sets the P0455 code.
Fix OBD-II code P0455: EVAP System Leak Detected (Gross Leak). Step-by-step diagnosis and repair guide.
- Read the diagnostic trouble code
P0456: The engine control module has sensed a tiny leak in the evaporative emissions (EVAP) system, usually less than 0.05 psi. The leak lets fuel vapors escape, triggers the check‑engine light, and can prevent the EVAP monitor from completing.
Fix OBD‑II code P0456: EVAP System Leak Detected (Very Small Leak). Step‑by‑step diagnosis, repair guide, cost guide, and safety tips.
- Check the gas cap
- Inspect visible EVAP hoses and connections
- Test the EVAP pressure sensor (P‑sensor) reading
- ...see all 5 steps →
P0496: The PCM has detected that the EVAP purge valve is allowing more vapor flow than expected during the purge cycle, indicating a possible stuck‑open valve or wiring issue.
Fix OBD-II code P0496: EVAP System High Purge Flow. Step-by-step diagnosis and repair guide.
- Check the gas cap and visible hoses
- Read live data for purge flow
- Test the purge valve resistance
- ...see all 6 steps →
P1457: The engine control module has sensed that the evaporative emissions (EVAP) control canister is leaking. Fuel vapors are escaping from the canister or its plumbing instead of being stored for later purge.
Fix OBD‑II code P1457: EVAP Leak Detected (Control Canister System). Step‑by‑step diagnosis, repair tips, cost guide, and safety notes.
- Check the Gas Cap and Visual Inspection
- Read Live Data for EVAP Pressure Sensor
- Perform a Smoke Test
- ...see all 5 steps →
SYMPTOM-FAILED-EMISSIONS: The powertrain control module has determined that the vehicle did not meet the required emissions standards during a recent test. It usually means a sensor, leak, or catalyst isn’t working within spec.
Fix OBD-II code SYMPTOM-FAILED-EMISSIONS: Failed emissions test — common codes and fixes. Step-by-step diagnosis and repair guide.
- Check the gas cap and obvious visual issues
- Read live data and freeze‑frame info
- Test the primary oxygen sensor (Bank 1 Sensor 1)
- ...see all 7 steps →
Understanding Powertrain Emissions Codes
Powertrain Emissions diagnostic trouble codes are set by your vehicle's onboard computer (ECU/PCM) when it detects a problem in the emissions system. Here's what you need to know:
- How codes are triggered: Sensors monitor emissions 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 emissions 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