DTC P1635: 5-Volt Reference 1 Circuit: Description

WARNING: This page is about a different car, the 2003 Oldsmobile Aurora. 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.

These 5-volt reference circuits are independent of each other outside the PCM, but are bussed together inside the PCM. Therefore a circuit condition on one sensor 5-volt reference circuit may affect the other sensor 5-volt reference circuits. The PCM monitors the voltage on the 5-volt reference circuit. If the PCM detects the voltage is out of tolerance, DTC P1635 sets.