P0120 Code — Throttle Position Sensor/Switch A Circuit Malfunction (Causes & How to Fix)

Quick 60‑Second Check

1. Look at the check‑engine light. If it’s flashing, the engine may be mis‑firing—don’t drive far.
2. Pop the hood and locate the throttle body. The TPS is a small, usually black, sensor mounted on the side of the throttle shaft.
3. Give the connector a firm wiggle. A loose pin will often cause P0120 instantly.
4. Scan with an OBD‑II reader. If the live data screen shows a flat “0.0 V” reading for the TPS, you’ve likely found the problem.

If any of those quick checks reveal a loose plug or dead sensor, you can move straight to the “Replace the throttle‑position sensor” step. Otherwise, continue with the full diagnostic flow.

What Triggers This Code

The ECM monitors the voltage coming from the TPS on Circuit A. Anything that prevents a clean, varying signal will set P0120:

• Broken or corroded wiring – heat, road salt, or rubbing against the intake can fray the harness.
• Faulty TPS – internal potentiometer wear, cracked housing, or water intrusion.
• ECM input failure – blown fuse, burnt pin, or internal module defect.
• Mechanical throttle problems – a seized throttle body can hold the sensor at a fixed position, confusing the ECU.

These are the root causes you’ll be hunting for in the steps below.

Symptoms You’ll Notice While Driving

These cues are often the first reason owners pull the code scanner, so keep them in mind when you’re test‑driving after the repair.

Step‑by‑Step Diagnosis & Repair

Below is the recommended order, from easiest to hardest. Follow each step exactly, noting what a good reading looks like versus a bad one.

1. Visual inspection of wiring and connector

• Good: Clean, tight connector; wires without cracks, chafing, or exposed conductors.
• Bad: Bent pins, corrosion, melted insulation, or a connector that wobbles when you tug.

Safety tip: Disconnect the negative battery terminal before pulling any harnesses to avoid accidental shorts.

2. Check for stored trouble codes and live data

• Plug the OBD‑II scanner into the vehicle’s data link connector.

• Select “Live Data” → “Throttle Position Sensor (A) Voltage.”

• Slowly press the accelerator from idle to wide open.

• Good: Voltage rises smoothly from ~0.5 V (closed throttle) to ~4.5 V (WOT).

• Bad: Voltage stays flat, jumps erratically, or spikes above 5 V.

If the live data looks normal but the code persists, move on to the next step.

3. Measure TPS voltage at the sensor

• With the engine off, back‑probe the two signal wires at the sensor (refer to the service manual for pin locations).

• Turn the key to ON (engine not started) and watch the multimeter.

• Good: Voltage climbs proportionally as you open the throttle by hand.

• Bad: Voltage stays at 0 V, stays at a constant high value, or flickers.

If the voltage is out of spec, the sensor is suspect.

4. Test TPS resistance (potentiometer type)

• Unplug the sensor and set the multimeter to Ω (ohms).

• Measure between the outer terminals (the two ends of the potentiometer).

• Rotate the throttle shaft slowly.

• Good: Resistance changes smoothly from a low value (≈0 Ω) at closed throttle to a higher value (≈5 kΩ) at wide open.

• Bad: Resistance is static, jumps, or reads “OL” (open circuit) at any position.

A faulty potentiometer will need replacement.

5. Inspect the ECM input circuit

• Locate the TPS harness at the ECM (usually a rectangular plug on the driver‑side firewall).

• Using the wiring diagram, test continuity between the sensor plug and the ECM pin.

• Check the related fuse (often labeled “ECU” or “TPS”).

• Good: Continuity passes (≈0 Ω) and the fuse is intact.

• Bad: No continuity, high resistance, or a blown fuse.

If the ECM side is compromised, a professional diagnosis is advisable because the module may need re‑programming or replacement.

6. Replace the throttle‑position sensor

• Tools: 10 mm socket, torque wrench (≈8–10 Nm for most vehicles).
• Remove the air intake duct if it blocks access.
• Unscrew the TPS mounting bolts (usually two).
• Pull the sensor out, being careful not to damage the throttle shaft.
• Install the new sensor, torque the bolts to spec, reconnect the electrical plug, and reinstall any intake components.

Safety tip: Never work on a hot engine. Allow the vehicle to cool for at least 15 minutes before removing the throttle body.

7. Clear codes and road‑test

• With the scanner still connected, select “Clear DTCs.”
• Start the engine, let it idle for a minute, then accelerate gently to 3000 rpm and back down.
• Re‑scan to confirm the P0120 code has not returned.

If the code reappears, double‑check wiring and consider the ECM as the next suspect.

Cost Breakdown

If the wiring harness needs repair, add $20 – $50 for connectors and $40 – $80 for labor. ECM diagnostics can push labor above $150, especially if a re‑flash is required.

Which Vehicles Are Most Affected?

• Toyota – especially older Camry, Corolla, and Tacoma models where the TPS sits behind the intake manifold.
• Honda – Civic and Accord (2000‑2010) often exhibit cracked sensor housings due to heat cycling.
• Ford – Focus, Fusion, and Ranger trucks built before 2015 have a known issue with the TPS harness rubbing against the firewall.
• Chevrolet – Silverado and Colorado (2007‑2014) owners report corrosion on the connector caused by exposure to road salt.

If you own any of these, pay extra attention to the connector condition during the visual inspection.

Can You Still Drive With This Code?

Yes, the car will usually run, but you’ll notice rough idle, hesitation on acceleration, and reduced fuel economy. Because the ECM may default to a “limp‑mode” strategy, power output can be limited, and the vehicle may fail an emissions inspection if the TPS signal stays out of range for a prolonged period.

Preventive Maintenance Tips

• Clean the connector every 30 000 mi with a contact‑cleaner spray and a soft brush.
• Inspect the throttle body for carbon buildup; a dirty throttle can cause the shaft to stick, pulling the sensor out of its normal range.
• Replace the sensor proactively if you notice a gradual loss of voltage smoothness, even before a code appears.
• Check the related fuse during routine maintenance; a marginal fuse can cause intermittent voltage drops that trigger P0120.

Related DTC Pages

• Learn why a P0122 (TPS Circuit Low) might appear alongside P0120: P0122 code
• Explore the P0102 (Mass Air Flow Sensor Circuit Low) for possible wiring cross‑talk: P0102 code
• See how a bad P0103 (MAF Circuit High) can affect throttle response: P0103 code

using these steps, you’ll be able to pinpoint whether the P0120 trouble originates from a simple connector issue, a failing throttle‑position sensor, or a deeper wiring/ECM problem. Armed with the right tools and a clear cost outlook, most DIY‑savvy owners can resolve the issue in under an hour and get back to smooth, responsive driving.

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