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P0121: Throttle Position Sensor (TPS) A Circuit Range Performance Problem

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What is P0121?

P0121 is a generic OBD-II Diagnostic Trouble Code (DTC) defined as “Throttle Position Sensor (TPS) “A” Circuit Range Performance Problem.”

Your vehicle’s Powertrain Control Module (PCM) or Engine Control Module (ECM) registers this code and activates the check engine light when it detects that the Throttle Position Sensorswitch A is operating outside the predetermined range. 

The throttle position sensor (TPS sensor) is a potentiometer located on the throttle body assembly to measure the throttle blade’s opening

Several old vehicles with a mechanical throttle body may have just one TPS sensor, while new makes and models come with two sensors (TPS “A” and “B”)  attached to the throttle body assembly. 

The TPS sensors send a reference voltage to your car’s PCM depending on your vehicle’s throttle position. At the idle position, the TPS voltage is 0.5 volts, and at Wide Open Throttle (WOT), the voltage is 4.5 volts. 

When your vehicle registers code P0121, it means that the throttle position sensor “A” is sending a reference voltage above or below its expected range. Some vehicles may also set code P0121 when the TPS signal doesn’t match up with the Manifold Absolute Pressure Sensor (MAP sensor) signals. 

Note: Vehicles with Electronic Throttle Control may use a pair of accelerator pedal position sensors attached to the gas pedal to control the throttle plate. In that case, the code P0121 may point to a problem with the accelerator pedal position sensor instead of the TPS sensor.

Common symptoms

Besides having an illuminated check engine light, here are the other symptoms associated with code P0121: 

  • Vehicle knocking or jerking 
  • The engine is hard to start 
  • Engine hesitates during acceleration
  • Engine stalling 
  • Rough idling 
  • The vehicle goes into the failsafe mode

Note: When in failsafe mode, the Powertrain Control or Engine Control Module cuts the current to the throttle actuator and puts the throttle valve at less than 6O opening. This way, the ECU limits your engine speed and restricts gear transmission to reduce the risks of your car’s system failures.                                                                                                                                                                                                                                         That said, the exact symptoms associated with the P0121 code may vary depending on your car’s engine control strategy for handling a system failure.

Can I still drive?

Code P0121 is a pretty serious check engine code as it will cause your vehicle to go into the failsafe mode. As a result, you may face several drivability issues like limited throttle response, rough idling, knocking, jerking, stalling, etc. 

If the root cause of the P0121 code isn’t diagnosed, your vehicle can develop more severe and possibly costly problems. 

So if your check engine light turns on due to the P0121 code, it’s best to stop driving your vehicle immediately and get the code diagnosed and fixed ASAP.

P0121 causes

Several issues could trigger your engine light for this trouble code, including a mechanical problem with the throttle body assembly. 

Here are the more common ones:  

  • A faulty throttle position sensor (throttle pedal position sensorswitch)
  • A defective or dirty throttle body
  • An open or short TPS circuit
  • Poor connection of the TPS connector
  • Corrosion on the sensor connector pins
  • A faulty pedal position sensor on the gas pedal
  • Fault in the pedal position sensor circuit 
  • Fluctuating MAP sensor voltage

Note: If your powertrain control module registers other codes along with DTC P0121, the causes may differ from those listed above.

Diagnosis

Code P0121 can be triggered on any vehicle, regardless of whether it’s a Volkswagen AG or a Toyota Motor Corporation model. It could be due to several reasons, from a dirty or faulty throttle body, a damaged connector to even a faulty MAP sensor. 

Here’s how a mechanic will diagnose the root cause of this trouble code: 

  • They’ll use an OBD-II scan tool to ensure no other trouble code is present. If other codes are found, they’ll address them first. 
  • Your mechanic will clear the code and test drive your vehicle to check if the code returns. 
  • They’ll then use the scan tool to read the idle and wide open throttle readings and compare them with the specifications given in your vehicle’s manual. If the readings are off, it indicates an issue with the throttle position sensor, its wiring, or its connector. 
  • They’ll visually inspect the sensor wiring and connector to check for any signs of corrosion. They’ll also check the wiring for any fraying or breakage. 
  • Next, they’ll remove the intake pipe from the throttle body to clean it. They’ll spray some throttle body cleaner on a rag and wipe all the carbon off the throttle plate and the body. 
  • If the code persists, they’ll use a multimeter or a scan tool to check the minimum and maximum voltage signal to identify any open or short TPS circuit A.

Possible repairs for P0121 & Costs

While P0121 is a generic powertrain fault code, the specific repair steps can vary depending on your vehicle’s make and model.

Typically, the repairs begin with an hour of diagnosis time, costing between $75-$150.

Once the root cause is diagnosed, your mechanic may suggest one of the following repairs:

  • Wiring repair/replacement: $100-$1000
  • Throttle body cleaning: $220-$290
  • Throttle position sensor replacement: $170-$230
  • Throttle body replacement: $580-$690

Note: The above estimates are inclusive of the part cost and labor charges. 

If code P0121 is accompanied by other trouble codes, a P0121 fix would need to be followed up with other repairs to ensure that the code doesn’t reappear.

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