Grand Prix Reduced Engine Power

I have a 2004 Pontiac Grand Prix GTP with about 140k miles on it that goes into reduce engine power mode occasionally. I think it only does it going up hill, but not every hill. Repair shop said throttle body $900. I replaced it myself as well as the accelerator pedal sensor (due to the codes that pop up) and I am still receiving the same codes. (P1125 and P2138)

What should be my next step?

Do you still have the reduced power or just the codes? Is your air filter clean?

Did you reset the codes after the repair or just expect the car to erase them? They need to be reset.

I have reset the codes with the reader. I haven’t checked the air filter myself, but my wife took it in for an oil change since this started happening. I would hope they’d check that. I’ll check tonight. I’ll get the reduced power and the codes now. “check stability” light has popped on when this occurs, but not every time.

Have a sunroof? Any water in the driver’s footwell? If so, take a look at this:

It does have a sunroof but I don’t feel any water inside the car. The air filter was clean.

Do you notice any engine pinging when going uphill? That can sound just like its name, a “ping” noise, often not very loud, that increases in frequency the faster the engine is going. Or if it gets worse, it can also sound louder, and like someone is rattling a metal soup can filled with nickles in the engine compartment. A quite noticeable clattering sound in other words.

Does your car use a ping or detonation sensor?

I haven’t heard any pinging. I’m not sure about a ping or detonation sensor.

Check wiring at pedal assembly, throttle body, and PCM.

Have you checked for an exhaust restriction?

Check that connector on the end of the dash on the driver’s side for moisture

My wife started backing out of the driveway and it started to shake. A new code popped up, along with the other two. P0128.

How would I check for exhaust blockage? I don’t believe the o2 sensors have ever been replaced. Curious if those and cat should be replaced for this?

With a vacuum gage.

I broke down and took it to a shop. They said I need to take it to the dealer and have them program the throttle body. Is there a way I can do this, or just have dealer do it? Also, does the accelerator pedal sensor need to be calibrated or reset?

Those fault codes relate to the accelerator pedal position sensor, nothing to do with the exhaust system. As mentioned before the wiring and connectors should be closely examined and tested, it may be worth the money to have an experienced technician repair this.

P0128 is related to the coolant temperature being too low, the thermostat may not be closing/sealing properly.

It appears to have been a defective accelerator pedal sensor in the one I replaced. Bought a pedal from the dealership and it’s been working so far (knock on wood).

That’ll teach me to buy car parts on Amazon…