P1631 on Chevrolet: How to Diagnose & Fix
Theft Deterrent Start Enable Signal Not Correct
1 Chevrolet model · 1 documented issue · 1 critical · $150-$600 repair
P1631 on Chevrolet by model: symptoms, diagnosis and fix
The Passlock anti-theft system causes intermittent no-start conditions where the security light flashes and the fuel pump is disabled. The ignition lock cylinder contains a Hall-effect sensor that reads a resistance value; when contacts wear or corrode, the BCM detects a mismatch and locks out fuel delivery. The problem typically worsens progressively from occasional to daily. 225+ complaints were filed for the 2002 model year alone.
Common Symptoms
- Security/theft light flashing on dashboard
- Engine cranks but will not start
- No-start condition occurs more frequently over time
- Car starts after waiting 10+ minutes
- SECURITY telltale illuminated
How to Fix
Sources for this issue (1)
P1631 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P1631 mean on a Chevrolet?
P1631 stands for "Theft Deterrent Start Enable Signal Not Correct." This is a manufacturer-specific (primarily GM) security code meaning the engine control module did not receive a correct 'start enable' signal or password from the vehicle's theft-deterrent / immobilizer system. The anti-theft system requires the ECM and the theft-deterrent module to exchange a matching coded handshake before the engine is allowed to run; if that signal is missing or doesn't match, the ECM blocks fueling and the engine may crank but not start. It generally indicates a communication or authentication breakdown in the immobilizer system rather than a mechanical engine fault, and is often accompanied by a lit security light. On Chevrolet it is documented across 1 model: Impala.
What are the most common causes of P1631?
The most common causes of P1631 are: Faulty or unprogrammed ignition key, key fob, or transponder, Worn or faulty ignition lock cylinder / key reader, Theft-deterrent module (passlock/immobilizer) fault, Damaged wiring or connectors between the BCM/TDM and ECM, Body control module fault, Module needing relearn after a battery, key, or part change, Signal interference from a second key on the ring or tampering. The specific cause varies by vehicle.
How much does it cost to fix P1631 on a Chevrolet?
Published repair estimates on Chevrolet range from $150 to $600 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P1631 on a different make?
View P1631 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.