For 1990-2009 cars

### Circuit Description

When the ignition is turned ON, the key is tested by the immobilizer anti-theft system. While the key code is being read by the immobilizer control unit, the engine can start and run with any key that will turn the lock cylinder. The key code is read and compared with key codes that have been stored in the memory of the immobilizer control unit. If a valid key is detected, the immobilizer control unit sends a serial data release message to the electronic control module (ECM). Included in the release message is an identification (ID) code which assures that neither the immobilizer control unit nor the ECM has been substituted to defeat the system. If the ECM receives an invalid release message, the ECM disables the fuel injector circuit.

### DTC Descriptors

This diagnostic procedure supports the following DTCs:

• | DTC U0100 Lost Communication with ECM/ transmission control module (TCM)

• | DTC U0167 Theft Deterrent Fuel Enable Signal Not Received

### Conditions for Setting the DTC

• | The ECM does not receive the signal from the immobilizer control module within 1.260 seconds after the ignition is turned on.

• | The above conditions are maintained until the ignition is switched OFF.

### Conditions for Clearing the MIL/DTC

The ignition switch is turned OFF or the scan tool TROUBLE CODE CLEAR command is issued.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Theft Deterrent System Schematics
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Connect the scan tool.
2. Turn the ignition ON, but do not start the engine.
3. Select immobilizer on the scan tool.

Is communication established between the scan tool and the immobilizer control unit?

Go to Step 3

Go to Scan Tool Does Not Communicate with CAN Device

3 | Read the IMMO & ECM ID CODE message that was displayed after requesting DIAGNOSIS. Did the message differ from normal message? Normal Message:

• | ECM MODE: LEARNT

• | IMMO. MODE: LEARNT

• | VIN CODE: SAME

Go to Programming Theft Deterrent System Components

Go to Step 4

4 | Check for an open serial data wire between the immobilizer control unit and the engine control module (ECM). Was the circuit open? | Go to Step 5 | Go to Step 6
5 | Repair the open serial data wire between the ECM and the immobilizer control unit. Is the repair complete? | Go to Step 8 |
6 |

1. Replace the immobilizer.
2. Reprogram the identification (ID) code. Refer to Programming Theft Deterrent System Components .

Is the repair complete?

Go to Step 8

Go to Step 7

7 |

1. Replace the ECM.
2. Turn the ignition OFF for more than 4 seconds.
3. Turn the ignition ON.

Does the DTC set?

Go to Step 8

8 | Operate the vehicle to verify the repair is complete. Do any DTCs set? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | System OK