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 an expected output voltage range, as does your TPS “B” Circuit (if so equipped). When code P0121 occurs, it means that your Engine Control Module (ECM) recognized that your throttle position “A” circuit’s expected output voltage has either gone above or below the sensor’s expected range.

What Are The Symptoms Of Code P0121?

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

What Is The Cause Of Code P0121?

  • Faulty Throttle Position Sensor (TPS)
  • Dirty Throttle Body
  • Faulty Throttle Body
  • An open or short of the TPS circuit
  • Bad connection of the TPS connector
  • Corrosion on terminal ends of electrical connections

How Serious Is Code P0121? – 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. In some cases, the car will not be capable of driving to the shop and will need to be towed.

Code P0121 Common Diagnosis Mistakes

Replacing the TPS or throttle body without attempting a throttle body cleaning.

Tools Needed to Diagnose Code P0121:

How To Diagnose And Repair Code P0121:

Difficulty of Diagnosis and Repair (3 out of 5)

  1. Check to see if there are any other codes along with P0121 and clear your Check Engine Light with FIXD.
  2. Check freeze frame data relating to the ECM to help pinpoint the exact issue.
  3. Check all TPS connections visually for loose or damaged items.
  4. Try cleaning the throttle body, this is an easy option that your car probably needs anyways. If there’s carbon build up on the throttle plate, it can cause the throttle to stick and send a faulty voltage. 
  5. Remove the intake pipe from the throttle body 
  6. Spray throttle body cleaner on a rag (this rag will get extremely dirty; don’t use one you can’t afford to throw away). Tech Tip: DO NOT spray throttle body cleaner into the throttle body. Spraying cleaner directly into the throttle body, instead of onto a rag, can damage the electrical components connected to the throttle body. 
  7. Open the throttle body with your hand using the pulley on the side if you have a cable operated system, or push on one side of the throttle plate to push it open.
  8. Wipe all the carbon (crusty black substance) off the throttle plate and the throttle body until its fully cleaned.
  9. In newer model cars with electronic throttle bodies, you may need to have a throttle body relearn procedure performed after this service.
  10. Now, test drive the vehicle and see if this fixed the concern.
  11. If it is still tripping the code, check for an open or short in the TPS sensor. You will need a multimeter to do this. Check out this video for a step by step, if you have a throttle cable system. If you have an electronic throttle body, you would need a diagnostic scan tool for further diagnosis. If the TPS fails this test, replace it.
  12. If it doesn’t, further diagnostics will need to be done by a reputable shop.

Estimated Cost of Repair

For error code P0121, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair. 

  • Throttle Position Sensor $170-$230
  • Throttle Body Cleaning $220-$290
  • Throttle Body $580-$690
  • Wiring repair/replacement $100-$1000

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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.

scott

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…..

Tony

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.

Max

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.

Tom

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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