If a malfunction occurs in a system monitored by the BCM, the BCM stores a diagnostic trouble code (DTC) in memory. DTCs are stored by the BCM as either current or history codes. A current DTC means that the fault was present when the diagnostic mode was entered. A history DTC means the fault occurred sometime after the DTCs were last cleared from the BCM memory. History codes are displayed even though the fault is not currently present. If a current DTC is stored the corresponding history code will always be stored. In the diagnostic mode the BCM displays any DTCs stored in memory. The DTCs are displayed as flash codes through the security indicator lamp on the instrument cluster (IPC). When working on any system controlled by the BCM the technician should always refer to this section and check for DTCs.

Perform the following steps to enter the BCM diagnostic mode:

The BCM will begin flashing trouble codes four seconds after entering the diagnostic mode. Each flash of the security indicator lamp on the IPC represents a number. For example, one flash followed by two quick flashes represents code 12. Each code is displayed three times before the next code is displayed. Stored DTCs are displayed in numerical order. Once the last code is displayed, the list begins again with the first code. The display continues until the diagnostic mode is exited.

When in the diagnostic mode, a change at certain BCM inputs will cause the BCM to respond to the change. For example, if either door lock switch is pressed the BCM will flash the fasten seatbelt indicator on the IPC and sound an audible warning tone. The BCM will also attempt to perform the operation normally performed by the switch operation. In this case, as long as the system is functioning properly, the BCM will lock the doors. This is an easy way for the technician to test the door lock actuators, switches and associated wiring without removing the door trim or components.

The BCM will also respond to a change at either shock sensor input. The tamper input is activated by minor blows to the vehicle. The shock input is activated by heavy blows. In the diagnostic mode either will cause the BCM to activate the horn relay and the horns will sound. The BCM will also flash the fasten seatbelt indicator on the IPC and sound an audible warning tone. An effective method of checking the shock sensor and associated wiring is to tap on the shock sensor while in the diagnostic mode.

Switching the interior lamps switch on and off while in the diagnostic mode will cause the BCM to flash the fasten seatbelt indicator on the IPC, sound an audible warning tone, and flash the courtesy lamps. On vehicles equipped with theft deterrent, each transition of the interior lamps command input will cause the BCM to step to next action in the following list:

1. Courtesy lamps flash
2. Horns sound
3. Exterior lights flash

In the diagnostic mode, diagnostic trouble codes (DTCs) are displayed in the following order:

1. DTC 12
2. Any current DTCs (DTC 21-DTC 25) or any history DTCs (DTC 31-DTC 35)
3. The remote control door lock transmitter diagnostic DTCs (DTC 41-DTC 45)

These DTCs will display if they are evoked while in the diagnostic mode.

4. The begin configuration display (DTC 55). DTC 55 and 2 configuration numbers are always displayed. The first number following DTC 55 is the hardware configuration. The second number is the software configuration.
5. After the configuration numbers are displayed, the entire list is repeated beginning with DTC 12.

DTC 12 is always set and displayed when the body control module (BCM) is in the diagnostic mode. DTC 12 indicates that the diagnostic function is in progress. DTC 55 is also always displayed.

If DTC 12 and DTC 55 are displayed when the BCM is in the diagnostic mode, and no other DTCs are displayed, the BCM is operating properly.

If DTC 12 and DTC 55 are not displayed when the BCM is in the diagnostic mode, refer to DTC 12 Diagnostic System Check .

If any other DTCs are displayed, refer to Diagnostic Trouble Code (DTC) List/Type .

System faults are stored as current or history DTCs. When working on systems controlled by the body control module, always refer to this section and test for DTCs.

History DTCs are codes that indicate that the BCM detected a fault which has disappeared.

Any DTC that begins with the digit 3 is a history DTC.

The fault will be stored as a history code under either of the following conditions:

If you cannot determine the cause of the failure through visual inspection, complete the following steps:

1. Connect a DVM to the suspected circuit.
2. Drive the vehicle.
3. Observe the readings. An abnormal voltage or resistance reading when the failure occurs indicates that the condition may be in the suspected circuit.