DTC P1631: Powertrain Control Module Detects Incorrect Password: Description
WARNING: This page is about a different car, the 2001 Cadillac Eldorado. However, it is still accessible from the selected car via links, so may be relevant.
Powertrain Control Module (PCM) controls fuel injector and starter operation based on Vehicle Theft Deterrent (VTD) password from Instrument Panel Cluster (IPC). When ignition switch is first turned to RUN position, IPC sends a programmed password to PCM. PCM acknowledges password and responds to IPC that normal fuel injector and starter operation will continue. If PCM detects an incorrect password, a theft deterrent system failure or an attempted vehicle theft, DTC P1631 will be stored. Engine will not start or crank as long as condition exists.