P0121 means Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance. The throttle position sensor A signal does not agree with what the computer expects from other inputs such as airflow and engine speed. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0121 means
Most vehicles built in the last two decades use electronic throttle control instead of a throttle cable: the accelerator pedal carries two or three position sensors, and the throttle body has a motor and two position sensors of its own. The computer compares the redundant sensors constantly, and any disagreement triggers reduced-power mode as a safety measure. Older engines with a throttle cable control idle with a separate idle air control valve.
P0121 symptoms
- Hesitation or surging
- Reduced-power mode on electronic throttle systems
- Erratic idle
- Harsh shifts on some automatics
Common causes of P0121
- Worn throttle position sensor
- Wiring or connector problems
- Dirty or sticking throttle body
- MAF or MAP faults affecting the comparison
How to diagnose P0121
- Watch the throttle position reading while slowly opening the throttle for dropouts
- Inspect the connector and wiring
- Clean the throttle body and recheck
Can you drive with P0121?
With care; sudden reduced power is possible. Repair soon.
P0121 tips
Before drive-by-wire, throttle position sensors wore a dead spot at the position used most, causing a stumble at steady cruise. Slowly sweeping the throttle while graphing the sensor on a scan tool shows any dropout clearly.
Will P0121 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0121 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0122 | Throttle/Pedal Position Sensor/Switch A Circuit Low Input |
| P0123 | Throttle/Pedal Position Sensor/Switch A Circuit High Input |
| P2135 | Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation |
Guides that help
When it is more than a sensor
Throttle and idle codes are fixed at the throttle body, the pedal, a sensor or a leak. If those check out and the engine still runs poorly, a compression test separates a control problem from a worn engine; compare engines by code on our engines page.
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