I have a 2002 Buick Regal LS. The engine idles normal on cold start. After the car is driven, the idle will be so high, it will accelerate the car. In neutral or park the idle will vary between 2k to 3k. Sometimes higher. With the throttle closed, the throttle position data is very erratic between 10 and 60 percent. But with KOEO, pressing WOT, it will read 100% The MAP data doesn’t make sense either. With KOEO MAP reads only 21 inHg. But when you press WOT it jumps back up to 29 inHg where it should be. When the car cools down, the MAP reads normal. Throttle is clean and replaced and reset IAC. No luck.
The P0121 is saying that the computer doesn't like the voltage signal it's receiving from the throttle position sensor.
Typically you'd suspect the throttle position sensor. But you have another issue with your MAP sensor voltages which can't be ignored.
The TPS and MAP sensor on your Regal share the same 5 volt reference from the computer. They also share the same ground.
So if your MAP sensor has an internal short (when it gets hot) it would give you those wonky MAP voltages AND it would drag down the 5 volt reference going to your TPS which would result in "out of range" voltage signals from the TPS (sensor) returning to the computer which would throw that P0121 code.
So you could grab a multimeter, (DC Volts) and on your TPS (sensor) connector you'll have a 5 volt reference, a Ground, and the TPS signal (going back the computer).
When your Regal starts acting up, measure that 5 volt reference at the TPS connector and see if it's being dragged down to a lower voltage.
If it is, unplug the MAP sensor and see if it comes back up to 5 volts.
If it does, then there you go.
Replace the MAP sensor.
If it doesn't, repeat the procedure but this time measuring the 5 volt reference on the MAP sensor connector. Is it being dragged down? Unplug the TPS (sensor).
Does the MAP sensor voltage come back up to 5 volts? Again, there you go. Replace the TPS (sensor).
You have to have that 5 volt reference going to both sensors (4.5 to 5 volts is fine anything less is no good).
Decided to pull out the PCM to have a look and C1 was badly corroded. I dont think the seals were installed correctly and moisture was allowed in. Cleaned C1 and C2 connectors on both sides and put it back together. Can’t replicate the problem anymore. Fixed.