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P0121 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


A P0121 trouble code is defined as when your vehicle’s throttle position sensor or switch A circuit is experiencing a range or performance problem.


Digging a little deeper into this trouble code, a P0121 is a generic OBD-II code that is registered by the engine control module. It means the ECM has seen that the TPS sensor circuit A output voltage is going above or below the expected range of the sensor. This is when it is compared to the TPS sensor circuit B.


There are a few primary factors in your vehicle that may cause a P0121 trouble code:

  • That the TPS has an intermittent up or an internal short.
  • That it is rubbing the harness and causing a short or open in the wiring.
  • There is a bad connection at the TPS.
  • That the PCM is bad (although this cause is less likely).
  • That there is corrosion or water in the sensor or connector.

Any one of these could trigger a P0121 trouble code to occur.


There are a number of symptoms that a vehicle may be experiencing, should a  P0121 trouble code be present with the vehicle:

  • The check engine light comes on.
  • The ECM will cause the check engine light to come on. This will put the vehicle in failsafe mode, which means that it will cut off the current to the throttle actuator, put the throttle valve at a little less than 6 degrees of an opening.
  • The ECM with control fuel injection and timing to control the speed of the engine.
  • You will be able to drive your vehicle, but only slowly and with very limited response to the throttle.
  • The engine may not accelerate as smoothly as before.
  • The engine may idle higher than it normally does.

Keep in mind that the symptoms that your vehicle is experiencing are determined by the manufacturer’s specific failsafe ECM strategy. This is something that is going to specifically correlate to a given manufacturer’s ECM strategy, which is why the symptoms can vary from make to make, and vehicle to vehicle.


As with many trouble codes, there is a specific series of steps that you need to follow to make sure you can properly diagnose the problem and the source of the problem.

  • First, you will need to scan and document all of the codes received on the OBD-II, here’s a great one we use everyday. View the freeze frame data to see when the P0121 trouble code was set.
  • Next, you will need to clear the fault codes and retest the vehicle to see if that trouble code comes back on your OBD-II scanner.
  • You will need to run a visual check of all the TPS wiring and connections for any damaged or loose items.
  • Finally, use your scanner to compare the data between TPS circuits A and B. If your results are incorrect or wrong, perform the TPS pinpoint tests that are laid out in the manufacturer’s recommendations.

If you follow these steps correctly to diagnose the problem, you should have no issues in correctly identifying and fixing this trouble code.

Common mistakes

However, there are some common mistakes that you should be aware of that even experienced mechanics can run into when investigating this trouble code.

The most common mistakes include:

  • Forgetting to scan and document the codes received, and not viewing the freeze frame data to properly see when trouble code P0121 was set.
  • Not clearing the OBD-II fault codes, then retesting the vehicle to verify that code and the same codes return.
  • Failing to run a visual inspection of all TPS wiring and connections for looseness and damage.
  • Failing to compare the data between TPS circuits A and B, using your OBD-II scanner.
  • Then if that information is incorrect, not testing to the manufacturer’s guidelines and recommendations.

How serious is this?

When you are having issues with specific problems related to a P0121 trouble code, the engine ECM will go into failsafe mode. This will cause a number of drivers’ complaints, per the issues with the vehicle that are mentioned above. This can include:

  • A lack of acceleration.
  • A high idle.
  • Jerking or bucking.
  • Limited speed.
  • Stalling.

All of which depend on the responses outlined by the manufacturer’s specific and unique failsafe mode.

What repairs can fix the code?

After you diagnose the problem, there are a few repairs that can fix it:

  • Repair the TPS connector.
  • Replace the TPS connector.
  • Repair the damaged wiring.
  • Replace the old TPS with a new sensor.

Related codes

None listed.

If your throttle position sensor fails, can you drive?

Driving with a bad throttle position sensor is not a good idea. If your car is not accelerating properly, or if it suddenly accelerates without you pressing the accelerator, driving in this condition can be risky.


Ongoing use of a vehicle when it is experiencing these issues can cause further problems and damage to the vehicle. We recommend that you take this to your manufacturer or a qualified mechanic as soon as you are aware there is a problem.

P0121 Code – What Does It Mean & How To Fix It