DTC P1639: 5-Volt Reference 2 Circuit: Description
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, 2002 Chevrolet RV Cutaway, and 2002 Chevrolet Cutaway. 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 2 circuit as a sensor feed for the Fuel Tank Pressure (FTP) sensor. The PCM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is out of tolerance, the PCM will set DTC P1639.