EngineLightCodes
Powertrain code

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

Severity
Fix soon
Typical repair cost
$20–$550
Do it yourself
Possible, medium
Shop time
0.5–2 hours

Sensor A is, in practice, throttle position sensor 1 at the throttle body; the accelerator pedal has its own codes (P2120 onwards).

How the engine computer detects it

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.

Written from public technical sources and not yet checked by a mechanic. How we write these pages

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)

Common causes

  1. Carboned or mechanically sticking throttle plate Most likely
  2. Worn potentiometer track (signal sticks or jumps) Possible
  3. Air leak or faulty MAF/MAP sensor (model comparison) Possible
  4. Contact resistance in signal or ground wire Possible
  5. Sensor or idle stop misadjusted (cable systems) Less likely

How to diagnose P0121

  1. Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
  2. Inspect the parts involved: throttle position sensor, throttle body, sensor wiring, sensor reference voltage.
  3. Work through the causes above from most to least likely, cheapest checks first.
  4. Clear the code and drive a full drive cycle to confirm the repair held.

Vehicles that report P0121 most

VehicleYearsReportsMedian miles
Chevrolet Traverse2010–2011374k

Counts of owner complaints to the NHTSA that mention P0121 (26 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.

Reading the code

P
Powertrain
0
Generic (SAE) code
1
Fuel and air metering
21
Specific fault

Questions drivers ask

Can I drive with code P0121?

Drive gently and only as far as you need to. The engine computer may limit power to protect the engine, so avoid highways and heavy traffic until it's fixed. If the check engine light starts flashing, pull over.

Will my car pass an emissions inspection with P0121?

Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.

Will P0121 clear by itself?

The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.

Can I fix P0121 myself?

Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.

Often reported together

Codes that owners mention in the same NHTSA complaints as P0121. They can point to a shared cause, so it's worth checking them together.

This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.