Immobilizer System: Operation

After the ignition is switched on, the engine control module sends a control signal and transponder read command signal to the immobilizer-ECU.
When the immobilizer-ECU receives the control signal from the engine control module through the key ring antenna, the immobilizer-ECU supplies a current and sends random number data to the transponder in the ignition key.
The transponder uses the random number data to derive an ID code, which is sent to the immobilizer-ECU through the key ring antenna.
The immobilizer-ECU compares the ID code that was sent with pre-registered ID codes, and if it matches, a control signal approving ignition is sent to the engine control module. If the ID code does not match (in the case of counterfeit ignition keys, for example), the immobilizer-ECU sends a control signal denying ignition to the engine control module, preventing the engine from starting.

Fig 1: Identifying Immobilizer System
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.