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.
The REDUCED POWER lamp will illuminate and the engine will be forced to shut down.
To locate an intermittent problem, use a scan tool to monitor DTC P1639 LAST TEST with ignition on, engine off. The scan tool will display FAILED if a short on the 5-volt reference circuit exists.
Driveability conditions will exist if a short on the 5-volt reference circuit is present.
Disconnecting sensors one at a time while monitoring the voltage of the remaining connected sensors on the scan tool will determine if a sensor is at fault.