P0121: Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance

Sensor A is, in practice, throttle position sensor 1 at the throttle body; the accelerator pedal has its own codes (P2120 onwards). With a cable-operated throttle there is a single potentiometer (about 0.5 V closed to 4.5 V wide open). Electronic throttles have two sensors sharing a 5 V reference and ground, usually running in opposite directions – on GM throttle sensor 1 falls from 3–4 V at idle to below 1 V at full throttle. P0121 is a plausibility fault: the signal is electrically valid but does not fit the engine's state. Manufacturers check this very differently. GM compares throttle position against air mass and pressures modelled from the other intake sensors and needs two trips; Saab/GM 2011 cut power. Ford used P0121 only on its older second-generation electronic throttle, for more than 7° disagreement between the two sensors, without MIL – newer Fords report that as P2135. Cable throttles (Saab 1997) check that idle voltage stays between 0.24 and 1 V. Carbon on the throttle and worn sensor tracks are typical causes.

Symptoms

• Check engine light on
• Limp mode with reduced power ('Reduced Engine Power') on electronic throttles
• Rough or too high idle
• Hesitation, delayed throttle response
• Shift problems on automatics (wrong load signal)

Most likely causes

• Carboned or mechanically sticking throttle plate
• Worn potentiometer track (signal sticks or jumps)
• Air leak or faulty MAF/MAP sensor (model comparison)
• Contact resistance in signal or ground wire
• Sensor or idle stop misadjusted (cable systems)

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the pedal and throttle connectors
2. Compare the reading with other engine data while varying the engine speed.
3. Use live data to confirm that both sensors rise smoothly and consistently
4. Clean the throttle if it has carbon buildup
5. Run the relearn procedure with the scanner if the manufacturer requires it
6. Replace the part identified by the diagnosis

Good to know

• Repair difficulty: medium
• It can trigger limp mode.
• It affects emissions.

What each character of P0121 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 1: Air and fuel
• 21: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0121?

With this code the car can suddenly lose power, so do not use it on highways or for passing. If it responds well, drive slowly straight to a repair shop; if it is already limited, have it towed.

What causes code P0121?

The most common causes are: carboned or mechanically sticking throttle plate; worn potentiometer track (signal sticks or jumps); air leak or faulty MAF/MAP sensor (model comparison).

Related codes

• P0122Throttle/Pedal Position Sensor/Switch A Circuit Low
• P0123Throttle/Pedal Position Sensor/Switch A Circuit High
• P0221Throttle/Pedal Position Sensor/Switch B Circuit Range/Performance
• P2135Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation
• P0068MAP/MAF - Throttle Position Correlation
• P0101Mass or Volume Air Flow Sensor A Circuit Range/Performance

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 18OBDG03A Engine Diagnostics
• newtonnet.co.uk: fordobd2012
• saabwisonline.com: dtc p0121 p0123 p0222 p0223 or p2135 with turbocha

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.