Get the app
Quick access from your home screen.
P1639 on a GM vehicle means the powertrain control module (PCM) has detected that its internally generated 5-volt reference circuit has dropped below the acceptable threshold. This reference voltage powers multiple sensors, so a low reading can disrupt sensor signals and set related codes.
Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
Get the app
Quick access from your home screen.