DTC P1635: 5-Volt Reference 1 Circuit: Description
WARNING: This page is about a different car, the 2001 GMC Savana, 2001 Chevrolet RV Cutaway, 2001 Chevrolet Cutaway, and 2001 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
Powertrain Control Module (PCM) uses 5-volt reference 1 circuit as a sensor feed to Throttle Position (TP) sensor, Manifold Absolute Pressure (MAP) sensor and Exhaust Gas Recirculation (EGR) valve pintle position sensor. PCM monitors voltage on 5-volt reference 1 circuit. DTC P1635 will set if voltage is out of tolerance.