Diagnostic Aids
WARNING: This page is about a different car, the 2006 Buick LaCrosse and 2006 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
DTC P1631 indicates that the PCM has received an incorrect password from the theft deterrent control module. This condition can occur if an incorrect key is being used when attempting to start the vehicle or if the BCM has been replaced and the PCM password learn function has not been enabled.