Please any help would be greatly appreciated. So my 2002 C5 would get the random reduce engine power p1120 and p1221 code . I would clear the code and my car would run fine after that. So i decided to get a replacement TPS (oem acdelco 17113566). Once replaced things got worse. I started to get p1120 ,p1221 and p1516. This also caused the car to idle very high and erratic. I checked all ground and verified the connections where not damaged. After disconnecting the battery for a few days the car is now only showing p1120 and p1221. I put back the old TPS sensor and was able to clear the codes and ran fine for a few minutes but then the car stalled out and i got p1120 and p1221 codes again. Any help or suggestion would be greatly appreciated on what to look for next. Could the new tps sensor be defective? could this be a TAC issue?

 

The P1120 (TPS1 Circuit) may be setting the 1221 which is a TPS 1-2 correlation code…TPS 2 may be OK but not good vs. TPS1…there are 2 potentiometers in the TPS sensor in which the signal voltages go in opposite directions from idle to WOT…if you have a DVOM and know how to use it we can look at the TPS 1 circuit….a 5 volt reference, a signal and a ground. A scan tool where you can graph the voltage is better but a labscope is the best !!…where was your OEM TPS purchased ??

 

I bought the TPS sensor from Ebay. I already reached out to them to either get a replacement or refund. So you think the TPS could be faulty and causing the p1221?

 

Like I had said if you have a DVOM and know how to use it you can check the 3 wires of TPS 1….this are the approximate voltages (pic below) you should see key on and at idle and at WOT..first check is check the 5 volt reference and ground !!…1.09 is idle and 4.32 is WOT…I would not buy any parts off EBay !!

 

The TPS should be from AC Delco, new. If memory serves, about $80.

The neat thing is that you can actually check your throttle actuation with the car "on" but engine off (so put it to the on position but don't start the engine.) This will let you take your time and test everything out, without the engine making loud distracting sounds. You should see your TPS dual-circuit (opposing) voltages go smoothly across the range, reverse to each other, as you press down on the throttle.

Do note, however, that there is more than just the TPS in this system. I had this issue before. The full set of components is this: APP (acceleration pedal position) sensor, TAC (throttle actuator control) module, TPS (throttle position sensor - the physical throttle, not the pedal), the throttle body itself, and the wiring between them all. I bring up the wiring because mine was faulty - it turned out to have been extended for a supercharger install, improperly.

The APP sensor is tied to the accelerator pedal and non-stock BUT there is an aftermarket solution. The TAC module is non-stock, but usually doesn't break. TPS is available, or was a couple years ago. Don't throw parts at it, but actually check all the components.

Ensure your TPS works by testing voltage (or, if you prefer, resistance with the module off your car.) Similarly, ensure your APP sensor works by testing voltage (or, if you prefer, resistance with the pedal off your car.) The APP has three circuits, similar to the TPS's two. With the modules on your car, ensure your throttle is in fact opening smoothly, and correctly, every time.

You may also find that a Tech2 (or clone/knockoff) can show you TPS/APP results in realtime for diagnosis and debug.

Let us know what you find.