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 comes on and the engine is disabled. The ignition lock cylinder carries a magnet read by a Hall-effect sensor in the lock housing; when the sensor, its wiring, or the BCM connector terminals fail, the BCM detects a mismatch and fuel is locked out. 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
Per GM guidance for 2000-2005 Impala, first inspect the BCM connectors for damaged or loose/unseated terminals before replacing the BCM, since many replaced BCMs test good. If the Passlock sensor in the ignition lock cylinder/housing is faulty, replace the lock cylinder/sensor assembly and perform the roughly 30-minute Passlock relearn (key on, engine off, until the security light goes out). As a temporary measure, leaving the key on until the security light stops flashing (about 10 minutes) and retrying will often start the car.
Sources for this issue (2)
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 →Join 600+ people using Au7o to better understand their vehicles.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.