P1631 — Theft Deterrent Start Enable Signal Not Correct (PassLock/PassKey)
STOPIs it safe to drive? Don’t keep driving — risk of real damage or a safety issue.
What P1631 means
This is a manufacturer-specific code — its meaning depends on the vehicle’s make (see below).
What P1631 means on specific makes
Chevy / GMC / Cadillac / Buick STOP
The start-enable password from the anti-theft system doesn't match what the PCM expects — so fuel/start is disabled. Classic GM PassLock no-start with a flashing SECURITY light. Often fixed by the 10/30-minute PassLock relearn or a failing PassLock sensor/ignition switch.
- PassLock sensor / ignition lock cylinder fault ($100-250)
- Needs PassLock relearn procedure ($0)
- BCM or wiring fault ($50-300)
- PCM was flashed and lost the password ($varies)
Check first: Do the PassLock relearn: turn key to ON, attempt start, leave key ON ~10 min until SECURITY light stops flashing, key OFF 20 sec, then start. Repeat 3x (30 min) if needed.
Source: engine-codes.com / RepairPal GM PassLock / Go-Parts