The vehicle theft deterrent (VTD) system functions are provided by the theft deterrent module (TDM) and the engine control module (ECM). 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 key is energized by the exciter coil surrounding the ignition lock cylinder. This exciter coil is part of the TDM. The energized transponder transmits a signal that contains its unique value, which is received by the TDM. The TDM then compares this value to a value stored in memory. If the ignition key is recognised as valid, the TDM performs a module substitution check where the identities of the instrument panel cluster (IPC), body control module (BCM), infotainment unit (IRC) and sensing diagnostic module (SDM) control units are checked. If they are recognised as valid components, the TDM will send the fuel enable password to the ECM via the BCM using the GMLAN. If the transponders unique value is incorrect or module substitution has been detected, the TDM will send the fuel disable password to the ECM. It is important to note that while it is referred to as the fuel enable/disable password, the vehicles starter may also be disabled, preventing engine cranking.

When the ECM receives the TDM fuel enable password, the ECM will challenge the password. The ECM sends this challenge back to the TDM via the BCM using the GMLAN. Both the ECM and TDM perform a calculation on this challenge. If the calculated response from the TDM equals the calculation performed by the ECM, the ECM will allow vehicle starting.

The components of the VTD system are as follows:

Vehicles with steering column mounted ignition switches have the exciter integral with the TDM, which is located within the steering column. The TDM can learn up to 10 keys (transponder values).

The TDM uses the following inputs: battery voltage, ignition switched voltage and ground circuit. The theft deterrent control module uses the following outputs: password exchange and challenge/response with the ECM.

When an ignition key is inserted into the ignition lock cylinder and the ignition is switched to accessories (ACC), the transponder embedded in the head of the key is energized by the exciter coils surrounding the ignition lock cylinder. The energized transponder transmits a signal that contains its unique value, which is received by the theft deterrent control module. The theft deterrent control module then compares this value to the learned key code stored in memory. If the ignition key is recognised as valid, the TDM performs a module substitution check where the identities of the IPC, BCM, IRC and SDM control units are checked. The theft deterrent control module then performs one of the following functions:

When the ECM receives the TDM fuel enable password, the ECM will challenge the password. The ECM sends this challenge back to the TDM via the serial data circuit. Both the ECM and TDM perform a calculation on this challenge. If the calculated response from the TDM 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 ignition key for Passkey III+ (PK3+) equipped vehicles is a standard ignition key with a transponder located in the plastic head of the key. The transponder value is fixed and unable to be changed. The VTD system uses the ignition key transponder value to determine if a valid ignition key is being used to start the vehicle. There are approximately 3 trillion possible transponder values. There are no visible electrical contacts.

Ignition keys have a black plastic head for full access operation of the vehicle. Ignition keys perform the following functions:

Module substitution detection is achieved by building an immobiliser chain made out of five control units which contain the VIN. When the ignition is switched to the ACC position, the TDM will request and compare the identities of each control unit to the identity stored in the TDM memory. This is called a module substitution check.

The following control units are in the immobiliser chain:

Two out of the four control units (IPC, BCM, IRC or SDM), are required to respond with the correct VIN to allow vehicle starting. Therefore, if only one unit is missing or reports with an incorrect VIN, vehicle starting is still enabled.