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2004 Pontiac Grand Am GT P1631 and U1000 Code After Using a Used PCM - Passlock Relearn Procedure & Diagnosis

Model: 2004 pontiac grand am gt Fault Code: P1631 Posted: 2015-04-24 19:08

After replacing the Powertrain Control Module (PCM) in my 2004 Pontiac Grand Am GT with a used unit, the vehicle will no longer start and now displays DTC P1631 (Passlock System Malfunction). The used PCM likely contains a passcode from another vehicle due to shared model year, engine type (3400), automatic transmission, and service number. I'm wondering whether performing a PassLock relearn procedure is sufficient or if additional steps are required. I have access to a Snap-On Solus scan tool for diagnostics. The original PCM was not flashed — it simply came from the same model year, make, and trim (3400 auto) with identical service number. I plan to perform the PassLock relearn procedure tomorrow and will update my results. I'm also now seeing a U1000 code (Class 2 Communication Error), which suggests potential issues with vehicle network communication or possibly a faulty PCM. Any insights on how to properly diagnose this would be greatly appreciated.

Related fault codes
P1631U1000
Comments (6)
Anonymous 2015-04-24 20:10

Did you flash the used PCM? If yes, perform a 10-minute relearn procedure: turn the ignition to 'ON' and then back to 'OFF'. Attempt to start the engine while keeping the key in the 'ON' position. After 10 minutes, observe the security light — it should extinguish. Turn off the ignition, wait 5 seconds, and then the vehicle will be ready for a new PassLock relearn on the next ignition transition from OFF to CRANK.

Anonymous 2015-04-24 22:34

No, the PCM was not flashed. However, it came from the same model year, make, model (3400 auto), and has the same service number as my vehicle. I will attempt the PassLock relearn procedure tomorrow and report back on the outcome.

Anonymous 2015-04-25 16:39

I completed the PassLock relearn this morning. The security light is now off, and the engine turns over — but it still won't start. After reconnecting the scan tool, I now see U1000 (Class 2 Communication Error). This makes me suspect a faulty PCM, though I'm not certain. Any suggestions on how to verify this or troubleshoot further?

Anonymous 2015-04-25 18:30

Use your scan tool to retrieve diagnostic codes from all modules — including BCM, ABS, airbag, TPMS, and others. Report back the full list of codes and note if any modules are unreachable by the scanner. This will help identify communication issues or hardware faults.

Anonymous 2015-04-25 19:06

No diagnostic codes are present when scanning all modules (BCM, ABS, airbag, TPMS, etc.). When I reinstalled the original PCM, the vehicle started immediately with no U1000 code. This strongly suggests that the used PCM had internal issues beyond just passcode problems — likely causing communication failures despite the relearn procedure.

Anonymous 2015-04-25 19:58

What specific issues do you observe in the used PCM? U1000 codes should be verified using a lab scope and a specialized diagnostic adapter to check for actual signal integrity or wiring faults, as standard scan tools may not detect underlying communication problems.