Immobilizer Code Word/ID Number Write Failure
The immobilizer system cannot save the security code needed to start your vehicle, like a lock that won't remember your key's pattern. This is typically a communication failure between the engine computer and the security module.
The ECU communicates with the immobilizer module to write and verify transponder ID codes during the key learning process. It monitors the success/failure of this data transfer handshake and flags a fault when the write operation times out or receives a NACK response.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Immobilizer Write Handshake Response Time | 50-200 milliseconds | Timeout >500ms or no response |
| Code Word Checksum Verification | Checksum matches expected value | Checksum mismatch or CRC error |
Code P1628 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1628 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.