P0121 – Meaning, Causes, Symptoms, & Fixes

Code P0121 Definition

Throttle Position Sensor (TPS) “A” Circuit Range Performance Problem

What Does P0121 Mean?

Your TPS “A” Circuit has a expected output voltage range, as does your TPS “B” Circuit. When code P0121 occurs, it means that your Engine Control Module (ECM) recognized that your “A” circuit’s expected output voltage has either gone above or below the sensor’s expected range when compared to “B” Circuit.

P0121 Symptom

  • Check Engine Light
  • Knocking or Jerking
  • Reduced top speed
  • Sluggish acceleration
  • Smoke during acceleration
  • Stalling

P0121 Causes

  • Difference in voltage output between “A” Circuit and “B” Circuit has exceeded 1.6 volts or has gone below .8 volts
  • An open or short of the TPS
  • Bad connection of the TPS
  • Corrosion in connector or sensor

Code P0121 Severity – High   

When code P0121 is triggered, your ECM will go into failsafe mode. Depending on the failsafe strategy of your car, this could lead to limited speeds or acceleration. This problem can also cause intermittent knocking, and even stalling for your vehicle.

Code P0121 Common Diagnosis Mistakes

Failing to conduct a visual inspection of all of the TPS connections to look for any loose or damaged items.

P0121 Diagnosis

 Diagnosis and Repair 

  • A sensor is needed for this next action, and the FIXD sensor is perfect for it. Scan and document the codes received in order to see when the P0121 code was triggered.
  • Check for an open or short in the TPS signal. You will need an oscilloscope to do this.
  • Check all TPS connections visually for loose or damaged items.
  • Compare data between TPS “A” Circuit and “B” Circuit.
  • Repair or replace TPS connector or sensor.

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Comments 9

Gary S.

September 26, 2018

The Regal was from a friend who wanted to sell it to me at a good KBB price. Didn’t buy it. Thanks FIXD!

David Vincent

June 28, 2019

My check engine light keeps coming on after replacing throttle position sensor. Should I clean throttle body? Please help.


December 19, 2018

Hi, I have a 2008 GMC 5.3. 143,000 klm.
If I have a full gas tank no engine light on. Once I get down to just under 3/4’s engine light come’s on.
PO121. Fill up and it goes away? I have no loss of power, no black smoke blowing out or any kind of a shifting problem? I have no problem with the truck at all but the light coming on with a shorter tank of gas? New gas cap?? What?

Aaron Rosenbaum

February 21, 2019

I also have a GMC 2008 5.3 that does the same strange thing…..did you ever figure out what goes on?
this is a bizarre kind of situation…..


June 9, 2019

Wow, I guess this is common on the 2008 Gmc Yukon. I also have one and ran across this while trying to figure out my issue. Love my vehicle but I might just fix it and let it go and I’m only at 71,000 miles.


May 4, 2019

I have a 2001 Volvo, out of the blue it had a lot of reduced acceleration, and an unresponsive throttle, then stalled on me while I was driving home one night. Super scary but it turned out okay for what it was. Wish I had FIXD before that though.

David Vincent

May 25, 2019

I just replaced this sensor and the check engine light has come on twice within the last two weeks (when I replaced it). Please help

Gerald Banks

June 23, 2019

David, did you find out the problem. I was told I needed to do the “Drive Cycle” so that the Engine light stops coming on.


October 4, 2019

Working on a 2009 Escalade Hybrid 6.0l and curious if the P0121 issue can trigger the P061B issue which is with the PCM. I know the PCM receives info from the Throttle Body so does anyone know if clearing up the P0121 issue will clear up the P061B issue? …or just thoughts lol. Thanks!

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