Throttle Position Sensor/Switch A Circuit Malfunction
The throttle position sensor (TPS) circuit is sending an out-of-range voltage signal to the engine control module. This prevents the PCM from accurately determining throttle position, which is critical for fuel injection timing and idle control. The code sets when the TPS voltage reading falls outside expected parameters.
What You Might Notice
- Check engine light illuminated
- Rough or unstable idle
- Engine stalling at stops or after starting
- Hesitation or misfiring during acceleration
- Poor fuel economy
Most Common Causes
- 1
Throttle position sensor failure
The TPS potentiometer becomes worn or damaged, causing it to output incorrect voltage signals. This is the most common cause of P0120.
- 2
Corroded or loose TPS connector
Oxidation or water damage on the TPS electrical connector creates high resistance, resulting in erratic voltage readings that exceed normal operating range.
- 3
Sticking throttle return spring
The mechanical return spring becomes seized or stuck, preventing the throttle from closing completely and causing the TPS to read incorrectly at idle.
- 4
PCM or wiring harness damage
A faulty engine control module or chafed/misrouted wiring harness disrupts the signal path between the TPS and PCM, causing voltage errors.
How to Diagnose It
- 1
Check Throttle position sensor failure — The TPS potentiometer becomes worn or damaged, causing it to output incorrect voltage signals. This is the most common cause of P0120.
- 2
Check Corroded or loose TPS connector — Oxidation or water damage on the TPS electrical connector creates high resistance, resulting in erratic voltage readings that exceed normal operating range.
- 3
Check Sticking throttle return spring — The mechanical return spring becomes seized or stuck, preventing the throttle from closing completely and causing the TPS to read incorrectly at idle.
- 4
Check PCM or wiring harness damage — A faulty engine control module or chafed/misrouted wiring harness disrupts the signal path between the TPS and PCM, causing voltage errors.
How to Fix It
- 1
Replace throttle position sensor
Remove the TPS from the throttle body, inspect the connector for corrosion, and install a new sensor. Verify the sensor voltage reading returns to normal range and clear the code.
- 2
Clean or replace TPS connector
Disconnect the TPS connector and inspect for corrosion or moisture. Clean with electronic contact cleaner or replace the connector assembly if damaged. Reconnect and test.
- 3
Inspect and repair throttle mechanism
Check the throttle return spring for corrosion or damage and verify it returns the throttle to idle position smoothly. Lubricate or replace the spring as needed.
- 4
Test wiring and PCM
Use a multimeter to test TPS wiring voltage at idle (typically 0.5V) and full throttle (typically 4.5V). If readings are correct but code persists, the PCM may be faulty and require replacement or reprogramming.
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