Code P0121 along with a random misfire?

Tiny
MEGANNELSON1008
  • MEMBER
  • 2004 CADILLAC DEVILLE
  • V8
  • AUTOMATIC
  • 58,999 MILES
I have the car listed above with a NorthStar engine v8. It's throwing code p0121 along with saying it has a random misfire. When I start it shakes a lot to the point where it looks like the engine is going to take off without the car. When driving it, it shuts off out of nowhere. It's burning fuel and you can even smell the gas. What could it be? The throttle body position sensor?
Sunday, October 5th, 2025 AT 10:21 AM

10 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Yep, a bad TPS can do this, here is the location of the sensor and how to change it out.

P0121 Throttle/Pedal Position Sensor “A” Circuit Range/Performance Problem

Perform the TP Sensor learn procedure:
1. Turn the ignition to the RUN/ON position.
2. Wait 1 minute.
3. Turn the ignition to the LOCK/OFF position.
4. Wait 15 seconds.

Clear the codes once the job is completed. Check out the images (below). Let us know how it goes.
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Tuesday, October 7th, 2025 AT 11:26 AM
Tiny
MEGANNELSON1008
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  • 10 POSTS
Okay, I've got the new TPS, so just unscrew the old one and hook the new one up?
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Thursday, October 9th, 2025 AT 2:35 PM
Tiny
KEN L
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Yes, and then do the relearn procedure above and it should fix the problem, make sure to clear the cods once completed. Let me know how it goes please.
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Friday, October 10th, 2025 AT 8:28 AM
Tiny
MEGANNELSON1008
  • MEMBER
  • 10 POSTS
I changes the TPS and relearn procedure as you told me to and unfortunately it didn't solve the problem. Once the codes were cleared I reran the obd2 and it said p0102 this time. So I took out the air filter and cleaned it (it was fairly new) and then took off the MAF sensor that was connected to the top of the air filter housing and cleaned the MAF and then cleared the codes again. Now the only code it's showing it a random misfire. Twice. But so when I start it, it shakes and it's rough and like idles real low one time it even didn't start and it's a new battery in it. But I've noticed every time I start it, it smells like gas outside the car so bad. Like you can really smell nothing but gas. So I reversed it and pulled it forward and noticed that when it's in reverse it's shaky a little but when I put it in drive, it shakes bad. Like it feels like the engine is going trying to jump out and take off without the body of the car. But when I put it in park to get out and look at it, when it's in park it doesn't shake like that.
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Friday, October 10th, 2025 AT 9:49 PM
Tiny
MEGANNELSON1008
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  • 10 POSTS
I can take a video and send it to you in the morning.
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Friday, October 10th, 2025 AT 9:49 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
I would remove the vacuum line from the fuel pressure regulator and if there is fuel present the regulator is bad and needs replacement, here is a guide to help and the location of the FPR in the images below.

https://www.2carpros.com/articles/why-an-automotive-engine-will-run-rich

Check out the images (below). Please let us know what happens.

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Saturday, October 11th, 2025 AT 9:25 AM
Tiny
KEN L
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Please respond
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Tuesday, October 14th, 2025 AT 12:59 PM
Tiny
MEGANNELSON1008
  • MEMBER
  • 10 POSTS
Ok I am going to try this today. It's been raining sorry
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Thursday, October 16th, 2025 AT 10:06 AM
Tiny
MEGANNELSON1008
  • MEMBER
  • 10 POSTS
Is this what I'm suppose to unhook if so there is no gas on it when I unplug it from that neck right there.
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Thursday, October 16th, 2025 AT 12:38 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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No worries, yes, that is what you are suppose to disconnect. So there is no gas in there correct? Lets remove the spark plugs for inspection and do a compression test as well. Please upload an image of the spark plugs so I can see them.

https://www.2carpros.com/articles/how-to-test-engine-compression

If you have a fuel injector stuck open and if you have low compression this will cause a rich mixture condition. Please go over this guide and get back to us.

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Thursday, October 16th, 2025 AT 1:17 PM

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