peterpeter211

I have a 2006 Covette Z-51with manual transmission and about 82,000 miles. Recently my check engine light (CEL) came on. Plugged in my scanner and saw that I have a P0121 diagnostic trouble code (DTC) for Throttle/Accelerator Position Sensor. I would like to share with the Corvette community my troubleshooting and repair steps. Hopefully my experience will help others.

Briefly the car has a throttle by wire system. There are two accelerator position sensors (APP) in the gas pedal. The powertrain control module (PCM) reads the voltage from the two APP sensors and generates a voltage to drive the throttle actuator controller (TAC). There are two throttle position sensors (TPS) that provide feedback to the PCM to let the PCM know the position of the throttle. The P0121 can indicate a problem with with either the TPS or the APP sensors.

At 82,000 the CEL came on. At the same time I noticed that occasionally when I accelerated from idle there was a slight pause between the time I pressed the accelerator and the engine responded.

I plugged in my Blue Driver OBD II scan tool and saw that I had the P0121 DTC. I set up to scan tool to view the scan for the two throttle position sensors (TPS), the throttle actuator control (TAC), and the two accelerator pedal position sensor (APP). The scan showed that when I pressed the gas pedal that the two APP scans and the TAC scan showed that the signal voltage increased from the base line in a smooth linear manner. In the first picture I attached you can see the APP and TAC have a smooth signal voltage increase. The scan also showed that the TPS had a momentary dip in signal voltage at the base line before the signal voltage increased in a linear fashion. In the first picture again you can see the dip in the signal voltage for Relative Throttle Position and for Absolute Throttle Position B when I first press the gas pedal. On the scanner, the freeze frame indicated that the P0121 fault was always set at idle. Also, on long freeway drives the CEL would clear and the scanner did not display the P0121 DTC.

I removed the TPS and it had a coating of carbon in the intake manifold side. probably from all the hours sitting in California traffic. (I assume that at low engine speeds that there is an overlap in the valve timing that permits exhaust gas to bleed past the intake valve back into the intake manifold which causes the carbon buildup.) I cleaned the TPS with a toothbrush and cleaner. After cleaning the TPS I measured the resistance across both sensors with a meter and an oscilloscope. The sensor appeared to be functioning properly.

After I installed the sensor the engine then had an idle problem. During closed loop operation, the engine would idle as high as 1,700 RPM. Again using the scanner I could see that the TAC was opening, even though the APP showed an idle signal voltage, and the TPS seemed to be operating correctly. When the car was stopped the idle would drop back to 700 RPM after a couple of moments. From a stop with the car in neutral the idle would increase if I let the car roll down a hill. The car idled ok when it was in open loop. A check with a local shop said that the idle problem was common with this model car and recommended reloading the software into the powertrain control module (PCM). That would cost me $115. However, after about 100 miles of driving the PCM appeared to have relearned the TPS position function and the engine idles as normal again and there is no CEL. I live in California and the car passed the California smog check today. The second picture I attached shows the scan with everything working. (This scan shows a slight irregularity at idle in Relative Throttle Position, Absolute Throttle Position B, and Commanded Throttle Actuator which was cause by idle fluctuations of about 100 RPM just before the PCM fully relearned the TPS position. The scan is completely flat now.) Now the TPS, APP and TAC all have a flat signal voltage at idle and a smooth linear increase in voltage as I press the gas pedal.

One last note. Before I cleaned the TPS I would often have the cruise control disengage on its own when I used it. Since I cleaned the TPS it had not disengaged again, unless I disengage it.

NEXT TIME: I think I will try cleaning the TPS without disconnected the electrical connector. Hopefully that will eliminate the idle problem after I reinstall the TPS.


First Picture Showing Dip in Voltage When Throttle Starts to Open For Relative Throttle Position and for Absolute Throttle Position B



Second Picture Showing Normal Throttle Scan

quickcat

My grand sport is throwing this code and this post helps me out a bit.

It is still under power train warranty but I am not willing to take the risk of paying for the diagnostic if the bad part is not covered. Running some data on torque like you did will help me figure out which part is really bad.

Thanks!

Matt

gtovett

This thread corresponds to the exact same issue I am having. P0121 code. I also did the cleaning of the throttle body with a small brush after disconnecting the electrical plug, replaced the gasket and reinstalled. Immediately the idle was high but finally settled down in park after 20-30 seconds or so.

While driving it is behaving exactly the same as OP noted. It won’t come down below 1100 rpm even when the speed and gear are low enough for the car to be at 700 or 800 rpm. When coming to a stop it still wants to keep the idle around 1100 as it downshifts (its an automatic transmission). It does finally settle down to 600 or so after being stopped for 20 seconds.

I did the idle re-learn procedure of 3 minutes at idle, then 1 minute engine off, then 3 minutes again at idle, then engine off, and it did nothing to help the problem.

I’ve driven over 200 miles since the throttle body cleaning and am out of ideas on how to fix this. It’s driving me nuts.

Any help???

Dcasole

Find someone with a GM TECH ll and do the throttle relearn procedure using the tool ...

I never heard of the procedure you used ...

Dave

gtovett

What is the procedure with a Tech 2? I called the dealer and described the issue and asked about them using a Tech 2 to re-teach the idle or a full ECM flash and they were confused why I was asking about that. Indicated that’s not what they do for this issue. She said that the car is just supposed to re-teach itself and they would check my work first thing. But obviously the car is not doing what it should. I decided not to take it in to them yet. $140 diagnostic fee to start with...

I took the TB off again last night, checked the gasket and plugged in the connectors with the negative battery cable off. Same result today. As soon as the car goes above 2mph it jumps to 1100rpm.

At this point maybe I just replace the Throttle Position Sensor? That was the original code in the first place: P0121. In the last 250 miles since I first cleaned the TB it has not thrown any codes.

This whole thing baffles me.

Dcasole

There is a relearn procedure .. I will have to look tommorrow for u and post screen shots . My ported throttle body was "hanging" and I did the procedure and it's fine now

Cherokee Nation

Where is the TPS located at?...I've was told years ago that it was in the TB.

Dcasole

It is ... and i did not think it was a servicable part , I thought u had to buy a throttle body ...

Dave

Cherokee Nation

Same thing i was told and you can't buy just the TPS and also what kind of tool do you use to remove the four bolts?

gtovett

For the TPS you would use a Torx security bit (since it has the little dimple inside) but If you cant service or replace just that part not sure what value there would be in removing it.



Dcasole, did you find that re-learn procedure you mentioned on Saturday?

Cherokee Nation

The OP said he cleaned the TPS..I don't think!! so,because that Torx tool is not available to guys like us...You have to replace with a new TB.

Dano523

Torx security wrenches, and a set is only like $5 on amazon.

gtovett

Hi Dcasole,

Have you been able to get that re-learn procedure you mentioned a few days ago? I'd really like to try it to see if I can resolve my high idle issue.

Dcasole

I did not have a chance to screen shot it this weekend and I am away on business till Friday .....I will try and do it as soon as I get home

Dave

Dcasole

OK here is the IDLE relearn procedure using a TECH ll ... ( sorry for the delay )

I am assuming all of you know how to pick your year make and model with a TECH ll so I will not be starting from the beginning ..

Once you are at the Diagnostics menu , chose POWERTRAIN



Next chose your engine and transmission





and now chose ENGINE CONTROL MODULE '


and then Module Setup



and then Idle relearn reset



then after looking over all the values to make sure nothing looks out of sorts ... click the reset at the bottom , you will hear the motor slow down then speed up and eventually it will settle in to the proper idle RPM commanded by the ECM





I hope this helps .....

Dave