Front Control Module (FCM)

U0021-CAN B BUS (+) CIRCUIT OPEN





For a complete wiring diagram, refer to the Wiring Information Electrical Diagrams.

- When Monitored:
With the ignition on and battery voltage between 10 and 16 volts.

- Set Condition:
The Front Control Module (FCM) detects the (D55) CAN B Bus (+) circuit is open.





1. TEST FOR INTERMITTENT CONDITION
1. Turn the ignition on.
2. With the scan tool, record and erase FCM DTCs.
3. Cycle the ignition from on to off 3 times.
4. Turn the ignition on.
5. With the scan tool, read active FCM DTCs.

Does the scan tool display this DTC as active?

Yes

- Go To 2

No

- The conditions that caused this code to set are not present at this time. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors.

2. ATTEMPT TO ISOLATE THE OPEN CONDITION




1. Turn the ignition on.
2. Verify that all CAN B Bus modules are communicating with the scan tool.

NOTE: A red "X" will be next to the module that is not communicating, indicating that the module is not active on the Bus network. A green check indicates that the module is active on the Bus network.

NOTE: If any module is not communicating, perform the appropriate no response test procedure before proceeding.

3. Turn the ignition off.
4. Gain access to the Front Control Module C1 harness connector, but do not disconnect.
5. Using a fused jumper wire, connect one end to ground and with the other end backprobe the CAN B Bus (-) circuit at the FCM C1 harness connector.
6. Turn the ignition on.
7. With the scan tool monitor the network status screen and document all modules that display a red "X".

Are there any red "X's" displayed next to the modules?

Yes

- Go To 3

No

- Check backprobe connection to ground, make sure it is proper. The CAN B Bus open DTC may no longer be active, it may be stored. Check all module connections
- Perform the BODY VERIFICATION TEST. Body Verification Test.

3. ATTEMPT TO ISOLATE THE OPEN CONDITION - MULTIPLE RED X'S
1. With the scan tool continue monitoring the network status screen.

Are there multiple red "X's" displayed next to the modules?

Yes

- The most likely cause of this condition is an open CAN B Bus (+) circuit between a common CAN B Bus splice and the modules that display the red X next to them. Use the wiring diagrams will help you determine where open condition exists.
- Perform the BODY VERIFICATION TEST. Body Verification Test

No

- Go To 4

4. (D55) CAN B BUS (+) CIRCUIT OPEN - SINGLE RED X
1. Turn the ignition off.
2. Disconnect the module that has the red X displayed next to it.
3. Turn the ignition on.
4. Measure the voltage between the (D55) CAN B Bus (+) circuit and ground.

Is there any voltage present?

Yes

- Inspect the connector and terminal for damage, inspect for spread terminals, or push out terminals. If ok, replace the module that displayed the red X next to it.
- Perform the BODY VERIFICATION TEST. Body Verification Test.

No

- Repair the (D55) CAN B Bus (+) circuit for an open between the next common splice and the module that has the red X displayed next to it. Use the wiring diagrams will help you determine where open condition exists.
- Perform the BODY VERIFICATION TEST. Body Verification Test.