DTC P1639: 5-Volt Reference B Circuit: Description

WARNING: This page is about a different car, the 2003 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) provides 5 volts to the following sensors:

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

These 5-volt reference circuits are independent of each other outside the PCM, but are bussed together inside the PCM. Therefore a circuit condition on one sensor 5-volt reference circuit may affect the other sensor 5-volt reference circuits. The PCM monitors the voltage on the 5-volt reference circuit. If the PCM detects that the voltage is out of tolerance, DTC P16391 sets.