Short answer
P0120 sets when the engine computer sees a throttle position sensor A voltage outside its normal operating range. Common causes are a bad sensor, connector corrosion, a chafed harness, a sticking throttle return spring, and, rarely, the computer. Check the 5 V reference and ground, then sweep the signal for glitches.
- Code
- P0120
- Generic title
- Throttle Position Sensor/Switch A Circuit Malfunction
- Meaning
- The signal is outside the range the computer expects.
- Severity
- Medium to high
What P0120 means
The throttle position sensor sends the engine computer a voltage that rises as the throttle opens. OBD-Codes.com describes P0120 as setting when the computer sees a voltage outside the normal operating range. It cites roughly 0.45 V at closed throttle rising to about 4.5 to 5 V as the throttle opens, but the exact range for your vehicle is in its service manual.
Symptoms
- Check engine light
- Misfiring at idle or at highway speed
- Poor idle quality, or no idle
- Starts and stalls
Causes
P0120 causes, per OBD-Codes.com
| Cause | How to check |
|---|---|
| Bad throttle position sensor | Sweep test shows a glitch, or the reading is stuck |
| Connector corrosion (TPS or nearby MAP connector) | Unplug and inspect for green corrosion and water |
| Chafed or misrouted harness | Wiggle test while watching live data |
| Sticking throttle return spring | Throttle does not snap closed |
| Engine computer | Only after the circuit checks out |
How to diagnose P0120
Watch the sweep
Key on, engine off, watch TPS voltage on a scan tool or meter while you open the throttle slowly. A glitch means a bad sensor; an intermittent glitch may only show on a scope.
If the reading is stuck or missing, unplug the sensor
Check for the 5 V reference and a good ground at the connector.
Test the signal circuit
With the reference and ground good, OBD-Codes.com suggests bridging the signal to ground at the plug and watching the reading: zero points to the sensor; no change points to an open or short in the signal wire.
Wiggle the harness
If idle changes when you move the TPS harness, suspect the connector or sensor.
Repair and relearn
Fix what failed, clear the code, and run any relearn your service manual lists.
More: how to test a TPS sensor, how to replace it, and the TPS codes hub.
Questions people ask
Is P0120 serious?
It can be. OBD-Codes.com lists misfiring, poor idle, failure to idle and stalling among possible symptoms. Treat a car that stalls or surges as unsafe to drive in traffic until it is fixed.
What is the difference between P0120 and P0121?
P0120 is a general circuit malfunction: the signal is out of its normal range. P0121 means the signal is within range but does not make sense for what the engine is doing.
Can a throttle return spring cause P0120?
OBD-Codes.com lists a sticking throttle return spring among the causes. If the throttle does not snap fully closed, the closed-throttle reading is wrong. Check that the throttle returns cleanly by hand on a cable throttle, engine off.
Sources
Last reviewed 2026-10-12. General guidance only: specs, procedures and fitment vary by vehicle, so confirm in your service manual and on the listing before you buy or wrench.