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P0121 error

I have a 2007 Yukon Denali (6.2l V8). Every so often I get a Check Engine Light. When I check the OBD II codes it shows P0121 (Throttle Position Sensor error). I can reset this error and may not get a Check Engine Light for weeks. When it does come on it seems to happen when my wife has been driving the truck (No, I’m not going there). I have replaced the air filter and cleaned the MAF. The engine runs and idles just fine. There has been no noticeable change in gas mileage or how the engine operates. Does anyone have any ideas?

Test the throttle position sensor to see if it’s operating properly.


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and if the TPS tests bad (or even if it tests good for that matter,) check the wiring going to the TPS to make sure it is good. I have seen a couple different Chevy/GMC’s have issues with the wiring for the TPS right at the TPS switch itself.

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How do you test the TPS?



For what it’s worth…

My wife has a “drive by wire” Impala. The Pedal assembly looks similar to yours. A few years ago it would intermittently store a pedal sensor code and light up “reduced engine power” and go into limp mode. I checked the (APP) Accelerator Pedal Position sensor, which is a double (redundant) sensor, wiring.

It was a bit like a puzzle, but I eventually pulled the right pin(s) and unclipped the right clips. Being down on the floor by wet snowy boots, I was looking for any corrosion, but saw none. I put some dielectric grease in the connection and put it back together. It never caused a problem, again.

I checked and the APP sensor/pedal assembly was somewhere around $100 (don’t remember, exactly) and fairly easy to replace, 2 bolts If I recall.

However, a while after I did that GM sent a letter saying they’d extended the APP sensor to 110,000 miles (if I recall). I called, but they would not do anything for me unless it was acting up when I gave them the car. They won’t take my word.

I have yet to replace it.


Here are some possibilities for the trouble:

A code P0121 may mean that one or more of the following has happened:
•TPS has intermittent open or short internally
•Harness is rubbing and causing an open or short in the wiring
• Bad connection at the TPS
• Bad PCM (less likely)
• Water or corrosion in connector or sensor

I would look at the sensor to make sure that you don’t have a bad connection to it. If that is okay then you might try replacing the sensor. Hopefully you don’t have a wire problem to the sensor causing the issue.

Fretting corrosion.

That’s what GM and others call a light corrosion in electrical connectors that can cause permanent or intermittent electrical problems.

I believe that’s what I was dealing with in my post, above. It is sometimes nearly invisible.

Sometimes unplugging a connection and reconnecting it solves the problem, but I like to use dielectric grease to keep the fretting corrosion from returning.

Here’s some information in a discussion…


Edit: another link-


Tester…Thanks for the link, it looks like an invaluable resource for amateurs like me, and from some of the misdiagnosis’s I have received over the years, some pros could benefit too.

Just curious, what motivated you to clean the MAF, I mean with a TPS diagnostic code? Did you ever have electric trains as a kid? If so you probably experienced dead spots develop on the transformer with use. Same thing can happen with the TPS. It gets used a lot, every time you press on the gas pedal.

What’s This Bob Pease Stuff Anyhow?

I cleaned the MAF because:

A. there was some crud on it.
B. it was suggested in a post I saw on the web.
C. It was easy.

Now it looks as though I’m going to spend some time crawling under the
dashboard to check the connections to the Throttle Position Sensor. It has
occurred to me that the error occurs when my wife drive the truck is because
we have the seat, mirror, gas pedal, position memory option. When she gets
in and selects her setting everything moves (she’s considerably shorter than
I am). So I’m starting to wonder if there is something loose. In fact I
may have knocked something loose when I replaced the Stop Light Switch. I
replaced it last summer because it was driving my electric brake controller
(for our travel trailer) nuts. The error started happening shortly after
that. :stuck_out_tongue:

The good thing about Car Talk is that it does make you think back a bit.

Re Fretting, it only reinforces what I learned decades ago: For signal (low power) circuits, use gold connections. For power circuits, use tin.

Of course, in automotive applications,we have to take wheat the designers give us.

Sounds like you are on the right track there OP. Best of luck. Everyone has a different driving style. I had a friend who’s driving style was really hard on wheel bearings. She was replacing wheel bearings all the time. A pothole ahead? No slowing down or avoidance maneuver whatsoever, same speed, straight head, bam. " Car’s are supposed to be able to take it " was her explanation … lol …

No offense, but the code is for the throttle position sensor circuit

You’re on the wrong track, digging around under the dash

The throttle position sensor is part of the electronic throttle body, which is under the hood

The accelerator pedal has an accelerator pedal position sensor attached to it, and those have separate codes, which you do not have

Again, please don’t get mad, but you’re looking in the wrong direction, under the dash

My gut feeling is you’ve got a problem with the electronic throttle body itself, or perhaps the pigtail. If you’ve got a good head on your shoulders, and a fluke to match, you should be able to figure it out. Beware, though, these problems are quite often intermittent, and if the truck is currently not acting up, you’ll likely not find anything wrong, when performing your tests

If it was me, I’d hook up that meter, backprobe the pins at the pigtail, and closely watch the screen, while you wiggle and tug on the wires. If the readings change dramatically, the pigtail is quite possibly the culprit. If everything is steady, then go ahead and buy a new electronic throttle body

Another note of caution . . . on this generation of Yukon, you typically need a factory-level scan tool to learn the idle stops after cleaning the throttle body. I expect it’s also a good idea to perform this test if you actually replace the throttle body. I have seen guys clean the throttle body and omit this step, and NOT get away with it, because the truck returned with an erratic and/or sky high idle.

GRoy’s got a tp sensor code, not an app sensor code

His problem is not going to be at the pedal

Under the dash you will find the Accelerator position sensor, the Throttle position sensor is in the Electronic throttle body. I don’t believe it is serviced separately.

** Throttle Position (TP) Sensor 1 Performance

This is a performance fault, not a voltage out of range fault. This means the computer detected a correlation problem between the sensors.

_ The predicted air flow and the predicted MAP combined are outside a calibrated range for more than 3 seconds .

The first set is to inspect for vacuum leaks that may throw the air flow values off. Next check the performance of the MAP sensor, a failing MAP sensor is sometimes the cause of P0121.

You can actually get the tp part of it separately, but it’s not worth the hassle. It’s almost the same price as the entire throttle body

After ignoring the Check Engine light (error P0121) for a number of months, I finally couldn’t stand it any longer. So after doing a little research on the web, I decided to clean the Throttle Plate Body. I removed it to clean both sides thoroughly and reinstalled it. I then went thru the ECM learning procedure recommended by GM and after a day and a half of driving around the ECM finally figured it out and set the idle to the right speed. The engine runs better and no more Check Engine light. I did not have to replace the Throttle Position Sensor. Yippee!!!


Please elaborate . . . exactly how did you perform the relearn?

No offense intended . . . but I’ve seen a number of guys clean GM electronic throttle bodies and run into problems later on, because they didn’t follow the correct procedure

Note that in some of these cases, the problems didn’t occur until a few days AFTER the throttle bodies had been cleaned. In all of these cases, performing the proper relearn procedure resolved the problems