Anti-theft immobilizer, On Board Diagnostic (OBD) (from May 1999) (Cabrio only): Function

WARNING: This page does not describe the selected car, but rather 27 other vehicles, including the 2002 Volkswagen Cabrio, 2001 Volkswagen Cabrio, 2000 Volkswagen Cabrio, 1999 Volkswagen Jetta, and 1999 Volkswagen Golf. However, it is still accessible from the selected car via links, so may be relevant.

The immobilizer of the 3rd generation implemented the "encrypted code" principle to increase anti-theft protection.

According to the "encrypted code" principle, the control module for anti-theft immobilizer randomly generates a number and transmits it to the "encryption" transponder in the ignition key.

Each transponder contains its own "fixed code". The transponder uses a specific algorithm to calculate a new number based on the transmitted number and the "fixed code".

This new number, the "encrypted code" is sent back to the control module for anti-theft immobilizer where it is backtracked according to the same algorithm.

The backtracked number and the original, randomly generated number must match. The key is then recognized as authorized.

Following recognition of the authorized key, a "variable code" is transmitted to the Engine Control Module (ECM).

The "variable code" is randomly re-generated in the Engine Control Module (ECM) after each engine start and is stored in both control modules.

If the "variable code" in the control module for anti-theft immobilizer and in the Engine Control Module (ECM) match, the vehicle can be operated.

For troubleshooting, initiate On Board Diagnostic (OBD) and read out stored data using the VAG1551 Scan Tool (ST).