DTC P1221: Throttle Position Sensor 1 To 2 Correlation: Description
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
The throttle body with throttle actuator control contains two throttle position sensors with independent 5-volt supplies and grounds. Each throttle position sensor has its own signal circuit which will vary inversely of one another. TP sensor 1 will range from 4.25 volts at idle to 0.59 volt at WOT. TP sensor 2 will range from 1.08 volts at idle to 4.41 volts at WOT. DTC will set if TP sensor 1 and TP sensor 2 signals are out or correlation with each other.