DTC P1635: 5-Volt Reference 1 Circuit: 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 Engine Control Module (ECM) uses two separate 5-volt regulated reference circuits to power the various three wire sensors on the vehicle. Five-volt reference circuit 1 supplies voltage to Throttle Position 1 (TP1), A/C pressure, Accelerator Pedal Position 2 (APP2) and the Fuel Tank Pressure (FTP) sensors. Five-volt reference circuit 2 supplies voltage to Throttle Position 2 (TP2), Accelerator Pedal Position 1 (APP1), and the Manifold Absolute Pressure (MAP) sensors. P1635 sets when the ECM determines that the 5-volt reference 1 circuit is outside a valid range.