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DTC P1631: Powertrain Control Module Detects Incorrect Password: Testing

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.
  1. If engine controls diagnostic system check has been performed, go to next step. If engine controls diagnostic system check has not been performed, perform ENGINE CONTROLS DIAGNOSTIC SYSTEM CHECK. See BODY CONTROL MODULE - ELDORADO article.
  2. Attempt to start vehicle. If vehicle starts, system is operating correctly at this time. If vehicle does not start, perform theft deterrent diagnostic system check. See THEFT DETERRENT DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM.