What is the P0120 trouble code?
The P0120 code is a generic OBD-II (On-Board Diagnostics) code indicating a malfunction in the Throttle Position Sensor (TPS) A circuit. This sensor, essential for proper engine operation, sends incorrect data to the Powertrain Control Module (PCM), which can affect vehicle performance.
Function of the Throttle Position Sensor (TPS)
The TPS is a potentiometer mounted on the throttle body. It measures the throttle plate’s opening angle and transmits this information in real-time to the PCM via an electrical signal. This signal allows the PCM to precisely adjust the air-fuel mixture and ignition timing.
The sensor is typically powered by a 5-volt reference voltage from the PCM and returns a variable signal:
- Throttle closed (idle): ~0.45V
- Wide Open Throttle (WOT): ~4.5V to 5V
The PCM can compare TPS data with that from the Manifold Absolute Pressure (MAP) sensor to check for consistency. An inconsistency can trigger the P0120 code.
Symptoms of the P0120 Code
Illumination of the Malfunction Indicator Lamp (MIL) is the most obvious symptom. Other signs may include:
- Misfires at idle or cruising speed
- Irregular or unstable idle
- Stalling, especially during acceleration
- Difficulty starting
- Noticeable decrease in performance and responsiveness
Possible Causes of the P0120 Code
Several components can be the source of this fault:
- Faulty TPS Sensor: The most common cause (internal wear, dead spot).
- Connection Issues: Corroded, loose, or oxidized TPS or MAP sensor connector.
- Wiring Problems: Damaged harness wires, chafing, short circuit, or open circuit.
- Mechanical Problem: Sticking throttle plate or its return spring.
- Faulty PCM: A rare cause, to be considered after ruling out all other possibilities.
Diagnosis and Solutions for Fixing P0120
⚠️ Warning: This information is for guidance only. For an accurate diagnosis, consult a professional mechanic.
1. Checking with a Diagnostic Tool
With the key in the ON position (engine off – KOEO), use a scan tool to observe the live TPS voltage. It should be around 0.45V at idle and increase progressively and smoothly to ~5V at full throttle. A stuck, erratic, or out-of-range voltage indicates a problem.
2. Visual Inspection
Inspect the TPS connector and wiring for obvious signs of damage, corrosion, or debris. Check the harness routing to avoid chafing.
3. Electrical Tests (DVOM)
Disconnect the connector and, with the engine off, check for the presence of the 5V reference voltage and a good ground. Test for continuity and the absence of a short circuit on the signal wire. Wiggle the harness during measurement to detect intermittent problems.
4. Replacement and Calibration
If tests point to a faulty sensor, replace it. Important Note: On some vehicles, the new TPS requires precise calibration (adjusting the voltage to 0.45V at idle). Refer to the specific repair manual for your model.
5. Diagnostic Conclusion
- If the TPS voltage becomes normal after replacement: problem solved.
- If the reading remains abnormal after replacement: check the wiring between the TPS and PCM.
- If the wiring is intact, an internal PCM issue is possible but rare.
Related Trouble Codes
Other TPS-related codes may appear: P0121, P0122, P0123, P0124.
Call to Action
Electrical diagnosis requires experience and the right tools. If you are unsure of your diagnosis, entrust your vehicle to a professional mechanic. They will have the necessary tools (oscilloscope, advanced scanner) to identify and resolve the problem effectively, saving you from unnecessary costly repairs.