Chevrolet P1631 Code: Theft Deterrent Start Enable Signal Not Correct
The engine computer received the wrong anti-theft password, so it blocks starting or fuel.
Stop driving
The vehicle usually can't be driven while P1631 is active, because the PCM won't enable the starter or fuel. Don't keep cranking; it drains the battery and makes the relearn harder.
At a glance
Severity: Moderate
- Meaning
- Theft Deterrent Start Enable Signal Not Correct
- System
- ECU & Outputs · Powertrain
- Safe to drive?
- No — stop driving
- Typical repair cost
- $0–$1,000
- DIY difficulty
- Easy
Each time you switch on the ignition, the Chevrolet body control module (or theft deterrent module) verifies the key and then sends a coded password to the PCM. The PCM compares it with the password it has learned; if the two match, it allows cranking and fuel. P1631 means a password arrived but didn't match. That's different from P1626, where the signal is missing altogether. The typical triggers are an unprogrammed or damaged transponder key, a BCM or PCM that was replaced or swapped without the password learn, or data corrupted by low voltage. It's found on PassKey-equipped models such as GMT800 Silverado, Tahoe and Suburban trucks, the C5 Corvette and the Cobalt and HHR. As long as the mismatch persists, the engine won't crank or will start and stall.
Try this first – 5-minute fix
- Try your spare key If a second programmed key starts the car, the first key's transponder is at fault. Keep other transponder keys and fobs away from the ignition when testing, as they can interfere.
- Charge the battery Make sure the battery is at least 12.4 V before trying again; a weak battery during cranking corrupts the password exchange.
- Perform the 30-minute relearn Turn the key to ON without cranking and wait until the security light turns off (about 10 minutes). Turn off for 5 seconds and repeat twice more for a total of three cycles, then start the engine. The PCM relearns the password from the BCM.
How to read P1631
- P Powertrain
- 1 Manufacturer-specific
- 6 Computer & outputs
- 31 Specific fault
Symptoms
- Engine won't crank or cranks but won't fire
- Engine starts briefly and shuts off
- Security light on or flashing, or a theft system warning in the driver information center
- Appears after a module replacement, a new key or a battery that went flat
- Other keys may start the vehicle normally
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Key not programmed to the vehicle or damaged transponder chip
- PCM or BCM replaced, swapped or reprogrammed without a password learn
- Low battery voltage or poor grounds corrupting the password exchange
- Faulty ignition key reader / antenna coil or its wiring
- Serial data communication problem between BCM and PCM
- Internal BCM or PCM fault
Diagnosis step by step
Read codes from the PCM and BCM
Use a scan tool that can reach the body control module. BCM codes for the key reader or transponder point to the key side, while a PCM-only P1631 after a module swap points to a missing password learn.
Tools: OBD-II scanner
Check battery and grounds
Measure battery voltage at rest (12.6 V ideal) and during cranking (should stay above about 9.6 V). Clean the battery terminals and check the engine and body ground straps.
Tools: Multimeter
Test with all keys
Try every key you own, one at a time with no other keys on the ring. If only one fails, replace and program that key; if all fail, the problem is in the vehicle.
Run the password learn
Perform the key-on relearn described above, or use a scan tool that has GM's theft deterrent learn function. This is required after replacing the PCM or BCM.
Tools: OBD-II scanner
Inspect the key reader and data wiring
If the relearn doesn't hold, check the transponder reader coil around the ignition lock and the wiring between BCM and PCM for damage or corrosion. A dealer or locksmith with GM software can confirm whether the key is recognized.
Tools: Flashlight, Multimeter
Where the parts are
- 1Battery
- 2Alternator
- 3Engine computer (ECM)
- 4CAN bus
- 5Transmission
- 6Wheel speed sensor
- 7High-voltage battery
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Password relearn procedure | Yes | $0–$120 |
| New transponder key cut and programmed | No | $80–$250 |
| Replace key reader / antenna coil | No | $150–$400 |
| Replace and program BCM or PCM | No | $350–$1,000 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Depends
P1631 concerns the anti-theft system. A US OBD emissions test only fails the car if the check engine light is on at the time of the test, and the car must of course run to be tested.
Frequently asked questions
Why did P1631 appear after I installed a used PCM?
The used PCM has learned the password of its original vehicle. It must learn your vehicle's password, either with the 30-minute relearn or with a GM-capable scan tool.
Is P1631 the same as P1626?
No. P1626 means the fuel enable signal wasn't received at all. P1631 means a signal arrived but its password was wrong, which points to keys or module programming.
Can a dead key fob battery cause P1631?
Usually not. The immobilizer transponder chip works without the fob battery. A physically damaged chip, however, can cause the password to fail.
How long does the relearn take?
About 30 minutes with the ignition on (three cycles of roughly 10 minutes). Keep a charger connected if the battery is weak.
Sources
- Manufacturer public service information (cross-checked)
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