DTC P1221: Throttle Position Sensor 1 & 2 Disagree: Description
WARNING: This page is about a different car, the 2002 Pontiac Firebird and 2002 Chevrolet Camaro. However, it is still accessible from the selected car via links, so may be relevant.
The Throttle Position (TP) sensor is mounted on the throttle body of the Throttle Actuator Control (TAC) assembly. The sensor is actually 2 individual throttle position sensors within 1 housing. Two separate signal circuits, a low reference circuit and a 5-volt reference circuit, are used in order to connect the TP sensor assembly and the throttle actuator control module. Each TP sensor signal voltage increases as the throttle opens. The signal circuit for TP sensors are pulled up to 5-volt reference voltage.