DTC P1639: 5-Volt Reference B Circuit: Description

WARNING: This page is about a different car, the 2002 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.

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

The Fuel Tank Pressure (FTP) sensor.
The A/C 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.