The immobilizer system functions are provided by the steering column module (CIM) and the engine control module (ECM). When a remote key is inserted into the ignition lock cylinder and the ignition is switched ON, the transponder embedded in the head of the remote key is energized by the theft deterrent module (TDM) exciter coil surrounding the ignition lock cylinder. This exciter coil is part of the TDM, which is secured directly to the CIM. The TDM contains no immobilizer logic; it only contains the exciter coils. All immobilizer logic is housed in the CIM. The energized transponder transmits an encrypted signal that contains its unique value, which is received by the CIM. The CIM then compares this value to a value stored in memory. If the values match, the CIM will send the prerelease password via serial data to the ECM. If the transponders unique value is incorrect, the TDM will send the fuel disable password to the ECM.

When the ECM receives the CIM prerelease password, the ECM will challenge the password. The ECM sends this challenge back to the CIM via serial data. Both the ECM and CIM perform a calculation on this challenge. If the calculated response from the CIM equals the calculation performed by the ECM, the ECM will allow vehicle starting.

The components of the theft system are as follows:

When an ignition key is inserted into the ignition lock cylinder and the ignition is switched ON, the transponder embedded in the head of the remote key is energized by the TDM exciter coils surrounding the ignition lock cylinder. The energized transponder transmits an encrypted signal that contains its unique value, which is received by the CIM. The CIM then compares this value to the learned value stored in memory. The CIM then performs one of the following functions:

Up to 5 remote keys may be learned to a single vehicle.

When the ECM receives the CIM prerelease password, the ECM will challenge the password. The ECM sends this challenge back to the CIM via serial data. Both the ECM and CIM perform a calculation on this challenge. If the calculated response from the CIM equals the calculation performed by the ECM, the ECM will allow vehicle starting.

The ECM will disable vehicle starting if any of the following conditions occur:

The remote key for the immobilizer system is a combination ignition key and keyless entry transmitter, with a transponder located in the plastic body of the key. The transponder value is fixed and unable to be changed. The immobilizer system uses the transponder value to determine if a valid remote key is being used to start the vehicle. There are approximately 3 trillion possible transponder values. There are no visible electrical contacts.

Certain modules use a security code as a means of theft deterrence. Each vehicle is assigned a specific four-digit security code at the time of manufacture. This security code is used by multiple modules which require a correct security code to operate properly. If a security code is not programming or an incorrect security code is programmed which does not match that of the vehicle, vehicle starting is disabled.

The following modules utilize security code for immobilization purposes:

When certain modules are programmed and configured during installation, the module learns a specific environment identifier which is unique to the vehicle. The environment identifier is used as a means to discourage vehicle theft by means of swapping modules between vehicles.

The steering column module (CIM) is the keeper of the environment identifier. The instrument panel cluster (IPC), underhood electrical center (UEC), and rear electrical center (REC) each learn the environment identifier during their configuration process. During vehicle operation, each of these modules report their environment identifier to the CIM. The CIM compares the received environment identifiers to its own stored identifier. If the identifiers match, vehicle starting is allowed. If two or more non-matching environment identifiers are received by the CIM, vehicle starting is disabled.