P0121
Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance
- Severity
- High
- Driving
- Drive only to a garage
Throttle or pedal position sensor "A" is producing a signal that is within its electrical range but does not agree with what the module expects from the other inputs. The sensor is not obviously broken - it is disagreeing, which is often harder to find and just as likely to trigger reduced power.
Possible causes
- CommonSensor drifting out of calibration with wear
- CommonCarbon build-up on the throttle plate
- PossiblePoor connection adding resistance to the signal
- PossibleVacuum leak making actual airflow disagree with throttle angle
- PossibleThrottle body mechanically worn or sticking
What you might notice
- Hesitation or surging under light throttle
- Reduced power mode
- Unstable idle
- Warning light on
What to check, in order
- Watch pedal position against throttle position on the live data screen through a slow full sweep of the pedal. They should track each other smoothly.
- Look for carbon build-up around the throttle plate, which stops it closing fully and makes the reported angle disagree with reality.
- Check for vacuum leaks - unmetered air makes a correct throttle angle look wrong.
- Inspect the connector for corrosion adding resistance.
- Check for other stored codes; a correlation code alongside this one narrows it to the sensor rather than the air path.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Fuel and air metering
P0100P0101P0102P0103P0104P0105P0106P0107P0108P0109P0110P0111P0112P0113P0114P0115P0116P0117P0118P0119P0120P0121P0122P0123P0124P0125P0126P0127P0128P0129P0130P0131P0132P0133P0134P0135P0136P0137P0138P0139P0140P0141P0142P0143P0144P0145P0146P0147P0148P0149P0150P0151P0152P0153P0154P0155P0156P0157P0158P0159P0160P0161P0162P0163P0164P0165P0166P0167P0168P0169P0170P0171P0172P0173P0174P0175P0176P0177P0178P0179P0180P0181P0182P0183P0184P0185P0186P0187P0188P0189P0190P0191P0192P0193P0194P0195P0196P0197P0198P0199