Possible causes

– A/C Pressure sensor harness is open or shorted
– MAP sensor harness is open or shorted
– EGR valve harness is open or shorted

When is the code detected?

The P1635 code is set when the reference voltage from the sensor is our of range.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P1635 GMC Description

The Powertrain Control Module (PCM) uses a common 5.0 Volt Reference 1 circuit as a sensor feed. This circuit supplies 5 Volts to the following sensors: The Air Condition (A/C) Pressure sensor, the Manifold Absolute Pressure (MAP) sensor and the Engine Gas Recirculation (EGR) valve. The PCM monitors the voltage on the 5.0 Volt Reference 1 circuit.

GMC Technical Service Bulletins for P1635 code