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  • P0121 stands for “Throttle/Pedal Position Sensor “A” Circuit Range/Performance Problem.” It’s when the modules detect that the throttle position sensor is operating outside a predetermined range.
  • The code is usually caused by a defective throttle position sensor, a faulty TP sensor circuit, or issues with the throttle body.
  • Some of its symptoms include ignition difficulties, engine stalling, and rough idling.

The error code P0121 may indicate an issue related to your engine’s throttle position sensor. Let’s take a look at code P0121 and its other potential consequences.

What Does the P0121 Code Mean?

Diagnostic trouble code (DTC) P0121 stands for “Throttle/Pedal Position Sensor “A” Circuit Range/Performance Problem.” The ECM/PCM stores this code (or some other similar codes) if the module detects that the Throttle Position Sensor is operating outside of a predetermined range.

throttle position sensor
A throttle position sensor operating outside of a predetermined range can trigger code P0121.

Also, on some vehicles, the code may be set when the TP and manifold absolute pressure (MAP) signals do not agree.

See also  P0122 Code: Throttle Position Sensor/Switch A Circuit Low Input

Note: Although code P0121 is a generic code specified by the Society of Automotive Engineers (SAE), the code’s definition may be different depending on the vehicle manufacturer.

What is a Throttle Position Sensor?

The throttle position (TP) sensor measures how much the engine’s throttle blade is open. Older vehicles with mechanical throttle linkage have just one TP sensor, whereas newer vehicles with electronic throttle control (ETC) have two. You’ll find the sensor(s) located on the throttle body assembly.

For an advanced, technical description of different configurations of throttle position sensors, as well as how the sensor can trigger an OBD code like P0121 read our discussion here.

You’ll notice that the code also mentions a pedal position sensor. This indicates that, on some vehicles, the code may point to a problem with the accelerator pedal position (APP) sensor, rather than the TP sensor. In an ETC system, the control module uses a pair of APP sensors as a primary input for control of the throttle motor/ throttle blade.

car throttle body
A faulty throttle body is one of the common causes of code P0121.

What Causes a P0121 Code?

Several factors contribute to the trouble code P0121. Below is a list of common causes.

See also  P0123 Code: Throttle / Pedal Position Sensor "A" Circuit High

Note: If there are other codes stored in addition to P0121, the causes and symptoms may differ from those listed here.

What are the Common Symptoms of the P0121 Code?

When code P0121 is set, you may notice one or more of the following symptoms:

  • The check engine light is on
  • The engine might be hard to start
  • The engine may hesitate during acceleration
  • The engine may stall
  • The engine may run rough or not idle correctly
  • The vehicle may be stuck in “reduced power” mode or fail-safe mode

How to Diagnose the P0121 Code

Because P0121 has many potential causes, it’s best to leave the diagnosis to a professional. But if you decide to troubleshoot the code yourself, the video below will give you an idea of what might be involved:

How to Fix the P0121 Code

Code P0121 has many potential causes—and many potential fixes. Some diagnostic work is needed to determine the correct route toward a proper repair.

See also  P2138 Code: Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation

With that said, only vehicle owners with sufficient technical knowledge should attempt DIY fixes on a vehicle with a P0121 code. Otherwise, bring the vehicle to the nearest auto repair shop and get a trained professional to repair the problem.

If you do decide to tackle the P0121 fix yourself, keep in mind that different types of vehicles may require their own distinct solutions—so always consult the appropriate repair information for your application.

Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.

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About The Authors
Richard McCuistian, ASE Certified Master Automobile Technician
Reviewed By Richard McCuistian, ASE Certified Master Automobile Technician

Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

CarParts Research Team
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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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