The Immobilizer enhances vehicle security by requiring the exchange of additional passwords before vehicle operation. If proper vehicle prerequisites have not been met prior to starting the engine, the body control module (BCM) will not forward the Continue Fuel message to the vehicle control module (VCM) even with proper passlock security codes. The BCM will signal the VCM (using class 2 serial data) that the proper authorization codes have not been received. The VCM will then shut down injector control resulting in a crank/no start condition.

The system is capable of arming itself within 20 seconds of the vehicle doors locking with a valid remote keyless entry signal. The arming operation must take place whether the driver uses the remote keyless entry key FOB, door lock switches, or if the driver fails to lock the vehicle. The system will arm instantly if it receives a LOCK command from a valid key FOB. If the driver locks the doors using the power door locks, or does not lock the vehicle at all after exiting, the Immobilizer will arm automatically in 20 seconds to prevent unauthorized starting attempts.

Should a fault in the system occur and the VCM interrupts fuel while the vehicle is operating, a Fail Enable mode will be activated. In this mode the BCM detected a problem after it has seen the correct access codes that may send the Fuel Disable message. In this mode of operation, the VCM will still allow fuel injection control. The SECURITY light should also be ON.

The remote keyless entry has a minimum of 50,000 discrete, rolling codes. Each time the key FOB button is pressed, the FOB sends a different access code to the receiver to enable fuel. this is done to prevent electronic recording of the transmitted codes.

The BCM sends the Fuel Continue message to the VCM only if the vehicle is started after a valid Passlock Key Cycle has occured. With the addition of the Immobilizer, the BCM must also receive a message that a Remote Driver Door Unlock command using a valid key FOB for that vehicle was performed. If the correct code is received the BCM will forward the Continue Fuel message to the VCM for 2 minutes. This message can be interupted by the following:

1. Immediately after receiving a valid LOCK command from a key FOB.
2. 20 seconds elapses after the ignition switch is turned OFF (if a remote UNLOCK command was not received within 2 minutes).
3. 2 minutes after receiving a valid UNLOCK command from a key FOB (timer times out).
4. 2 minutes after receiving a valid override code and/or completing key FOB programming (refer to Key FOB Programming).

For normal Immobilization operation to occur, the ignition switch must be in the LOCK, UNLOCK, or ACC position; or any ignition switch position where IGN 1 is not active. As the UNLOCK command is issued from a valid key FOB, the BCM responds by enabling (but not sending) a Fuel Continue password for 2 minutes.

When the ignition switch completes a full ignition cycle and returns to the CRANK position, the BCM responds be sending the Fuel Continue password to the VCM, provided valid passlock and remote keyless entry key FOB messages were received and monitors for an answer back. The BCM continues to send the Fuel Continue password every 50 msec during the first minute after IGN 1 is active to the VCM and once every 10 seconds thereafter until it receives a Vehicle Security Status message from the VCM. Once the BCM receives the Vehicle Security Status message from the VCM, it responds with a Vehicle Security Status Acknowledge message.

If the correct code is received by the BCM from a valid key FOB, the BCM tells the VCM to enable injector drivers and the vehicle will start. If a Vehicle Security Status message is not received by the BCM witin 1 minute of IGN 1 becoming active., DTC U1255 is set and the SECURITY lamp is illuminated. If an invalid remote keyless entry key FOB code is received by the BCM, it will not allow the VCM to enable the injectors. The BCM interprets an incorrect code as a possible tampering with the system and flashes the SECURITY telltale. At this point the BCM will not accept a valid code for up to 256 seconds.

There are 3 states of operation for the Immobilizer. They are as follows:

1. ARMED
2. DISARMED
3. ARMED DELAY

An override code is provided to disable the immobilization function when a key FOB UNLOCK command is not or cannot be used to gain entrance to the vehicle. It must also be used to allow programming of the key FOB (s) to the BCM. The override code consists of five digits with values of 1-9. An override code, which varies between different BCM's is entered into the BCM during assembly. The override code can be entered manually (refer to owners manual for instructions) or through the use of a scan tool.

Override of the immobilization system is accomplished using the TECH 2 as follows:

1. Select F1: BODY in System Selection Menu.
2. Enter vehicle options as applicable.
3. Select F4: IMMOBILIZER in Application Menu.
4. Select F2: SPECIAL FUNCTIONS in Application Menu.
5. Select F4: CHANGE OVERRIDE CODE in Application Menu to change the current code number and follow directions displayed by the TECH 2
6. Select F5: OVERRIDE PROCEDURE in Application Menu to bypass the immobilizer for 2 minutes so the vehicle may be started or the key FOB's can be programed.