DTC P1635: 5-Volt Reference Circuit "A" Or No. 1: Description

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2002 Pontiac Grand Prix, 2002 Pontiac Bonneville, 2002 Chevrolet Monte Carlo, 2002 Chevrolet Impala, and 2002 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.

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

Throttle Position (TP) sensor.
Manifold Absolute Pressure (MAP) sensor.
Exhaust Gas Recirculation (EGR) valve pintle position sensor.
Engine oil pressure sensor (except Regal).
Fuel Tank Pressure (FTP) sensor (except Bonneville and Regal).

PCM monitors voltage on 5-volt reference circuit. DTC sets when voltage is out of tolerance.