DTC P1635: 5-Volt Reference 1 Circuit: Description

WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, 2002 Chevrolet RV Cutaway, and 2002 Chevrolet Cutaway. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) uses the 5-volt reference 1 circuit as a sensor feed to the following sensors:

The Throttle Position (TP) sensor.
The Manifold Absolute Pressure (MAP) sensor.
The Exhaust Gas Recirculation (EGR) valve pintle position sensor.

The Powertrain Control Module (PCM) monitors the voltage on the 5-volt reference 1 circuit. If the voltage is out of tolerance, the PCM will set DTC P1635.