DTC P1635: 5-Volt Reference 1 Circuit: 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 Powertrain Control Module (PCM) uses the 5-volt reference 1 circuit as a sensor feed to the following sensors:

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.