Immobilizer Control Unit

The immobilizer control unit is an electronic module in the instrument panel which verifies the validity of an ignition key when the ignition is turned ON.
To accomplish its purpose, the immobilizer control unit performs the following actions:
- Learns and stores the codes of valid keys.
- Reads the radio frequency input from the ignition key.
- Compares the received code with the codes of the valid keys.
- Sends a release message to the powertrain control module (PCM)I Engine control module (ECM) if a valid key has been presented.
- Calculates and transmits identification (ID) codes within each release message.
- Controls the status indicator in the instrument cluster.
- Monitors system faults.
- Supports system test functions.

Normal Operation
When the ignition is turned ON, the immobilizer control unit tries to read the key code transmitted by the transponder in the ignition key.
If a valid key is detected, the immobilizer control unit sends a release message to the PCM/ECM. The release message contains an ID code. Immobilization will be performed by the PCM/ECM if no release message is received, or if the ID code in the PCM/ECM does not match the immobilizer control unit ID code.
If a non-valid key is detected, the release message is not sent to the PCM/ECM.

Data Link Connector (DLC)
When the ignition is ON, a scan tool can switch the immobilizer control unit to the data link connector (DLC) mode for the purpose of diagnostics, key coding, or ID coding.

ID Code Handling
One of 65,535 possible ID codes is stored in the immobilizer control unit's memory.
The ID code can be erased by using the scan tools' RE-SET ID CODE command. When the immobilizer control unit calculates a new ID code, the PCM/ECM ID code must be reset to match the immobilizer control unit ID code. To reset the ID code, refer to "ID Code Reprogramming."
During diagnostic procedures, the ID code can be read for comparison with the PCM/ECM ID code by using the scan tools' READ IMMOBILIZER CONTROL UNIT ID-CODE command.