Looking for serious input. Having an issue with my 2004 Supercharged Monte. The check engine light continues to trip with the same code (P0121) for TPS Throttle Position Sensor. Its the only code being generated - no others. I have put two new TPS sensors on the car since last Saturday. Each time I clear the ECM codes. When the car is cold and sitting all night, everything works fine for the first 15-20 miles of driving when started, but after the entire drivetrain gets fully hot from normal driving, the check engine light trips again (stays on steady) and at that same time the transmission stops up-shifting correctly. Once the check engine light trips from the TPS code, as the trans goes through each gear, the engine rev's way too high (3500, 4000, 5000 rpm). Typically the only way I can get it to up-shift and grab the next higher gear is if I gently ease off the gas pedal. The transmission seems fine and strong in every way and I do not believe the problem exists in it, but that TPS sensor seems to be sending a dirty signal after reaching full operating temperature (20 minutes of driving) and then the trans doesn't like to up-shift. The original (old) TPS sensor did this, so I put the 1st new one on last weekend. The 1st new sensor acted the same way after a half-hour of driving. I cleared the code and it generated another TPS code within 10 miles of driving. Suspecting I had a bad sensor out of the box, I put a 2nd new TPS sensor on last night, cleared the ECM code again, then took it for another ride. The check engine light tripped again with the same TPS code while test driving with the 2nd new sensor - and the transmission stopped up-shifting again. I have an appointment next week at a reputable repair center but would like to solve this on my own.

Has anyone ever experienced this phenom before? Any ideas what else could be interfering with the TPS sensor signal and causing it to send the code and fail to provide a clean signal to the ECM? The plug and connector is clean, tight and wires in good shape. What other sensors work in conjunction with the TPS that may not be generating their own code, but instead causing the TPS signal to be faulty?


Any thoughts & experiences out there??

 

How does the engine run otherwise (say in park)? This code is saying the engine speed the pcm is seeing doesn't correlate to where it sees the tps + iac positions. TB blade damage, IAC damage can cause this (allowing air past causing the engine to idle high and the engine can't get the speed down). A larger vac leak can also cause this. A good general check for the latter is to check your map sensor reading as well as fuel trims. If the map reading is off and the trims are adding a bunch of fuel, this means air is entering after the MAF. Its usually fairly big though as this code means its impacting engine speed by enough % to trip this mismatch code.


If the engine is running perfectly fine, then Id deep dive the signals in/out of tps. Make sure the 5v reference is present. Then check output relative to spec, including throughout the full range (key on engine off of course) - it should linearly increase. If you get to this point, Id bet you'll find something while monitoring TPS voltage - either its dropping to zero, erratic, etc due to wiring issues.

 

Thank you for your input. The engine actually runs excellent. Smooth under power, steady at idle - and the Supercharger is hitting its marks too. The engine idles smooth & steady at about 700-800 rpm with no indication of irregularity. The only thing I can say, is when the 1st new TPS acted the same as the original, I had a local station use their scanner to investigate. He said it was just the lone TPS code and he looked at other things and felt everything else looked good. He went through the range of throttle motion with the key on and engine off.... said there were no flat spots in the feedback signal.

The only other thing, which may or may not even be related... is when I come to a stop after long driving, pause for a few moments in drive, but at idle (like when I pull up to my mailbox)... when I shift into reverse and press the pedal, the engine likes to stall. It always restarts easy, but this is the only phenomenon I experience with engine performance and it may not even be related. Reason unknown.

As for the MAF sensor, it's been cleaned (very gently/carefully) just as a precaution and there are never any MAF codes being generated. I'm half tempted to go buy a 3rd new TPS sensor from a different store chain and get a different brand, just to see if I've encountered a flakey manufacturing lot of bad TPS sensors from Autozone.

 

I would try a different brand. It happened to me with a certain brand of o2 sensors. Went through 4 or 5 before we realized its was just that brand.

EDIT: I believe my problem brand was 'Walker'.

 

To clarify, I didn't mean to imply the MAF was bad, that's definitely a different set of codes/ issues. Its just that I have seen a car with a vac leak after the maf that tripped this code (along with a couple others). The engine couldn't hit its target idle to match the actual TP% because the leak was letting in a good bit of air (fueled by heavily skewed trims so it ran fairly ok besides the obviously wrong fuel trims and map value). So it thought the TPS was wrong and that the throttle was actually partly open. But if yours is idling correctly and running good otherwise then it cant be that.

Certainly dropping a 3rd one in wouldn't hurt, but Im skeptical it'll help. Assuming that doesn't help - do you have a way to log data either via an OBD2 scanner or a multimeter? It's clearly not happy with that output signal line, so we really need a way to capture the data before/during/after to see what the pcm is seeing that makes it throw a flag.

Sometimes electrical weirdness can be very short lived so having a data log you can replay frame by frame can make it easier to spot. So trying to watch it live while driving, you may not catch it.


Another random question since you suspect getting up to temp does it - will it still do it if it warms up sitting there at idle in park? A pinched/chafed wire or a loose connection is a potential here - actual driving/ bumps could be the trigger. If it doesn't do it sitting there / not moving, then perhaps that could give another clue.

 

Thank you for yours/everyone's input. All good points. I'm comfortable ruling-out a vacuum leak. The engine runs too perfect otherwise. Like new. Unfortunately, I don't have a way to run/check/log diagnostics realtime during a drive. I have not tried letting the car reach operating temps by sitting at idle, though that is a good idea. Here's a new twist... yesterday while driving, the check engine light went out on its own. This almost never happens. After it went out, I drove the car all evening for over 100 miles and it never came back on. Throughout the evening, the engine ran flawless and the transmission shifted flawless, even using cruise control/passing gear, etc. I have an appointment to drop the car off on Monday morning. In addition to all the background info, I think I'll tell them to put a 3rd new TPS on of their brand choice and explain everything. My gut is telling me this is still temperature related to the sensor, even though it drove well last night. Its definitely flakey. I'll see how it acts this morning when I drive another 100 miles. I've had so many aftermarket parts be bad out of the box over the years that my trust is low. This shop I have the appointment with has done tens of thousands $$$ in repairs on all four of my Monte SS's over the last 15 years and the owner thrives on troubleshooting these ellusive electrical matters. All of his mechanics have done great work for me over the years, but the owner tends to interecpt my work orders and do the repairs himself for some reason. Maybe I'm picky, idk, but I trust them. We'll see what happens when I drive it today.