Diagnostic Aids
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
To locate an intermittent problem, use scan tool to monitor DTC P0645 LAST TEST with engine running. Wiggling wires while watching for a change from NOT RAN or PASSED to FAILED may locate the area where an open in the wiring may exist.
A/C relay coil resistance: 70-115 ohms.