DTC P1639: 5-Volt Reference 2 Circuit: Description
WARNING: This page is about a different car, the 2002 GMC Savana and 2002 Chevrolet Chevy Express. 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.