DTC P1221: Throttle Position Sensors 1 & 2 Correlation: Description
WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
The throttle actuator control assembly has 2 Throttle Position (TP) sensors mounted within the assembly. The Powertrain Control Module (PCM) provides individual signal, ground, and 5-volt reference circuits to each sensor. Both sensors operate within a voltage range between 0.35-4.65 volts. When the throttle is opened from zero to 100 percent, one sensor signal voltage increases while the other decreases. The signal circuit for TP sensor 1 is referenced to ground, and the signal circuit for TP sensor 2 is pulled up to 5 volts within the PCM.