If it is the same problem I had, it is in the transmission and requires the pulling of the transmission. This is a comman problem with some Montanas. It might be the pressure control solenoid located inside the valve body of the transmission. Yes this is probly your answer. These cars have this problem alot. I work for Gm and I know first hand. I was having the exact same problem two weeks ago. I replaced the pressure control soleniod and the transmission now shifts as smooth as it did brand new.
The throttle position sensor is located on the throttle body at the end of the throttle shaft.
It has to be replaced. no adjustments.
The throttle position sensor can be removed by dismantling the chassis in which it sits. This can be done by referencing the owner's manual f the Pontiac Montana.
The Throttle Position Sensor is conneceted to the end of the Throttle Shaft inside the Throttle Body. P.S. The Throttle Position Sensor is part of the Throttle Body and must be replaced as a unit. Hope this helps ya.
Bad Throttle Position Sensor
The throttle position sensor determines the amount of throttle used when the engine is running at various speeds. If the sensor is faulty, it can be replaced by ordering a new part.
The throttle position sensor is actually apart of the throttle plate actuator assembly and must be replaced as a unit.
The Throttle Position sensor measures the throttle position, which is controlled by the gas pedal. It is used to determine engine load and if it fails it can cause automatic transmission shifting problems.
It is in the throttle valve body since these are fuel injected and no carburated. A google search shows what one looks like. Symptoms are hard shifting with high idle to just a normal than high idle. Either way it should be replaced as it will fail and saves on prper fuel milage and running of engine. It fastens to the throttle body by 2 bolts and is a common fix.
possibly the throttle position sensor the sensor will also change the shifting pattern
The OBD code p1221 indicates a bad throttle position sensor. Sometimes cleaning the throttle body will clear this code. If that does not work the sensor will need to be replaced.
It's part of the throttle body, entire unit must be replaced.