For 1990-2009 cars

### Immobilizer Description and Operation

The remote function receiver module provides a coil which is installed on the column lock. This coil is used to read out the transponder code of the key. The information is sent to the body control module (BCM).

The main immobilizer function is performed as an interaction between several modules. The process that determines whether the engine may be engaged or not consists of several steps. First: The keys used for the vehicle must be registered in the BCM. Second: In several modules a vehicle identification code is programmed. The participating ECUs are electronic brake control module, HVAC control module, instrument cluster and inflatable restraint sensing and diagnostic module. At least three of these modules have to answer within a given time to enable an engine start, an answer from the instrument cluster is always required. Third: Between the engine control module ECM and the BCM a security data exchange is required also to enable an engine start. The ECM sends a challenge to the BCM, that sends a calculated value back to the ECM. If the calculated values are correct, engine start is permitted.

If any of these three steps fails, the engine will not start by disabling starter, fuel injection and spark. In addition, an indicator indicates the error.