DTC P1639: 5-VOLT REFERENCEWide Open Throttle (WOT)CIRCUIT: Description
WARNING: This page is about a different car, the 2002 Pontiac Montana, 2002 Oldsmobile Silhouette, and 2002 Chevrolet Venture. However, it is still accessible from the selected car via links, so may be relevant.
The PCM uses the 5-volt reference "B" circuit as a sensor feed for the A/C refrigerant pressure sensor. The PCM monitors the voltage on the 5-volt reference "B" circuit. If the voltage is out of tolerance, the PCM will set DTC P1639.