P1629 Anti-Theft System Communication Failure
P1629 is a manufacturer-specific diagnostic trouble code (DTC) commonly found in General Motors vehicles. It indicates a failure in the anti-theft system, specifically a communication issue between the engine control module (ECM/PCM) and the immobilizer module or steering column. The security system (Passlock, Pass-Key III, or VATS) cannot authenticate the key's transponder or detects a corrupted security signal. As a result, the ECM disables the ignition or fuel injectors to prevent the engine from starting, triggering a security lock state and illuminating the 'Security' or 'Check Engine' light.
01. Most Common Causes
- Faulty or missing transponder in the ignition key
- Defective immobilizer or Passlock module
- Faulty ignition cylinder or steering column sensor
- Short or open circuit in the security system wiring
- Loss of communication between the anti-theft module and PCM (data bus)
- PCM replaced or reprogrammed without performing the security system relearn procedure
02. Symptoms You Will Notice
- Engine cranks but does not start
- Security or Check Engine light illuminated on the dashboard
03. Step-by-Step Diagnosis
Scan for DTCs and verify P1629 is present
Check for communication between the PCM and anti-theft module using a scan tool
Inspect the ignition key for a functioning transponder
Test the ignition cylinder and steering column sensor for proper operation
Check wiring and connectors for damage, corrosion, or shorts
Perform a security system relearn procedure if the PCM was replaced
1. Frequently Asked Questions
Is it safe to drive?▼
No, the vehicle may not start or may stall while driving due to the security lock, posing a safety risk. Immediate diagnosis and repair are recommended.
How much does it cost to fix?▼
Repair costs typically range from $100 to $500, depending on the root cause (e.g., key replacement, module repair, or wiring fix).