DTC P1635: 5-Volt Reference A Circuit: Description

WARNING: This page is about a different car, the 2002 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) uses a common 5-volt reference A circuit as a sensor feed. This circuit supplies 5 volts to the following sensors:

The Manifold Absolute Pressure (MAP) sensor.
The engine oil pressure sensor.

The PCM monitors the voltage on the 5-volt reference A1 circuit. This DTC sets if the voltage is out of range.