Fault in the theft deterrent (immobiliser) system, such as a failed or unrecognised key transponder or a problem reading the key; Wiring or connector fault in the cranking signal circuit between the ignition switch, body control or theft module and the PCM (open circuit, short, corrosion, poor pin tension); Worn ignition switch or faulty ignition lock cylinder or key reader not providing the correct cranking signal; Low battery voltage or poor earth or power connections causing unstable signals during cranking; Faulty body control module, theft deterrent module or other module in the immobiliser communication path; Faulty PCM or a security data mismatch after PCM or module replacement that has not been relearned.

P1629

Theft Deterrent System Cranking Signal.

Buick
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
6: AreaComputer and output circuits
29: FaultThe specific fault in that area

What it means

P1629 on a Buick means the powertrain control module has a fault with the theft deterrent system cranking signal. The PCM expects a valid cranking or start-enable signal linked to the vehicle theft deterrent (immobiliser) system while the engine is being cranked, and it did not receive one or received an implausible one. The exact circuit and the modules involved depend on the model and year.

Symptoms

Engine cranks but does not start, or starts and then stalls shortly after
Engine does not crank at all in some cases
Security or theft warning light on or flashing in the instrument cluster
Intermittent starting problems that come and go
Check engine light on with the code stored

Possible causes

Fault in the theft deterrent (immobiliser) system, such as a failed or unrecognised key transponder or a problem reading the keylikely
Wiring or connector fault in the cranking signal circuit between the ignition switch, body control or theft module and the PCM (open circuit, short, corrosion, poor pin tension)likely
Worn ignition switch or faulty ignition lock cylinder or key reader not providing the correct cranking signalpossible
Low battery voltage or poor earth or power connections causing unstable signals during crankingpossible
Faulty body control module, theft deterrent module or other module in the immobiliser communication pathpossible
Faulty PCM or a security data mismatch after PCM or module replacement that has not been relearnedrare

How to diagnose

Read all stored codes and freeze frame data, noting any other theft, body control or starter related codes and when the fault was set
Check battery condition and charging voltage, and inspect battery terminals and main earths, as low voltage during cranking can upset the signal
Watch the security light behaviour with the key on and during cranking, and try a second key to rule out a faulty key or transponder
Inspect the ignition switch, key reader and related connectors for damage, moisture, corrosion or loose pins
Check the cranking signal circuit wiring and connectors between the ignition switch, theft or body module and the PCM for continuity, shorts and voltage drop, using the manufacturer wiring diagram for the specific model
Use a scan tool to view theft deterrent and security data, including whether the PCM and security module agree and whether the cranking signal is seen during cranking
Check for any applicable service information or software updates for the model and year
If wiring, key and switch checks pass, test or substitute the body or theft module and then the PCM, following the model-specific procedure

How to fix

Repair or replace damaged wiring, connectors or terminals in the cranking signal circuit
Replace a faulty ignition switch, lock cylinder or key reader, or a damaged key or transponder
Restore good battery condition and clean or repair power and earth connections
Replace a failed body control, theft deterrent or other module as confirmed by testing, then perform the required security relearn
Perform the theft deterrent or PCM security relearn procedure if modules have been replaced or data does not match
Replace the PCM only after all other causes are ruled out, then program and relearn it correctly

Can I keep driving?

Medium severity

If the car starts and runs normally it can be driven for now, but the fault may leave you stranded with a no-start, so get it checked soon. If it is stalling or struggling to start, avoid relying on it for journeys.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.