P1628 Immobilizer Code Write Failure
The P1628 code indicates that the engine control unit (ECU) has detected a failure when attempting to write or update the immobilizer code word or ID number in its internal EEPROM memory. This process is critical during key adaptation, ECU replacement, or immobilizer module replacement. If the write fails, the ECU cannot validate the key, preventing the engine from starting.
01. Most Common Causes
- Internal EEPROM memory failure in the ECU (defective memory cells or wear)
- Interruption of power supply (low battery or loose connection) during the adaptation/writing process
- Communication error on the data bus (CAN or K-Line) between the ECU and the immobilizer module (cluster or separate module)
- Corrupted ECU software or incompatible version with the coding procedure
- Repeated failed key adaptation attempts that lock the writing process for security reasons
02. Symptoms You Will Notice
- Engine cranks but does not start
- Immobilizer warning light illuminated on the dashboard
03. Step-by-Step Diagnosis
Verify battery voltage is stable and above 12.5V; charge or replace if low
Check for other immobilizer-related codes (e.g., P1629, P1630) and communication codes (U-codes)
Inspect wiring and connectors between ECU and immobilizer module for damage or corrosion
Attempt to reprogram the immobilizer using a professional scan tool; if write fails, suspect ECU or immobilizer module fault
If reprogramming fails repeatedly, test the ECU's EEPROM integrity or replace the ECU
1. Frequently Asked Questions
Is it safe to drive?▼
No, the vehicle will not start, so it is not drivable. This is a high-severity issue that requires immediate repair.
How much does it cost to fix?▼
Diagnosis typically costs $100-$200. Repair costs vary: ECU reprogramming $150-$400, ECU replacement $500-$1500, immobilizer module replacement $200-$600, plus labor.