Possible causes

– Sensor using 5 volt reference line shorted to ground
– Circuit short to ground

When is the code detected?

If the voltage is out of tolerance, the PCM will set DTC P1639.

Symptoms

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

P1639 GMC Description

The powertrain control module (PCM) uses the 5-volt reference 2 circuit as a sensor feed for the A/C refrigerant pressure sensor. The PCM monitors the voltage on the 5-volt reference 2 circuit.

GMC Technical Service Bulletins for P1639 code