The error code P0121 may indicate an issue related to your engine’s throttle position sensor. Like other throttle position sensor-related codes, this could lead to serious problems and can result in your vehicle not starting at all. 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.” Your car’s primary computer—also known as the powertrain control module (PCM)–monitors the operation of the TP sensor(s). If the module detects that the sensor is operating outside of a predetermined range, error code P0121 will be set.
Also, on some vehicles, the code may be set when the TP and manifold absolute pressure (MAP) signals do not agree.
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.
The PCM uses data from the TP sensor when calculating functions, such as idle speed, fuel delivery, and automatic transmission shift points. Also, on vehicles that have ETC, the PCM (or another control module) uses data from the TP sensors as a primary input for control of the throttle motor/throttle blade. This type of system uses two TP sensors as a fail-safe measure.
Code P0121 is particularly serious on vehicles with ETC because it usually causes the control module to force the vehicle into failsafe mode. This inhibits the car’s speed and acceleration.
In some cases, you might not even be able to drive your car once this happens.
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.
What are the Possible Causes of the P0121 Code?
Several factors contribute to the trouble code P0121. Below is a list of common causes.
- Defective throttle position sensor
- Fault in the TP sensor circuit
- Problem with the throttle body
- Problems with other sensors (e.g., MAP and MAF sensors)
- Issues with the control module (e.g., software in need of update)
- Faulty APP sensor
- Fault in the APP circuit
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.
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.