Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Diagnostic Test
- VERIFY THE DIAGNOSTIC TROUBLE CODE (DTC) IS ACTIVE NOTE: Ensure the IOD fuse is installed and battery voltage is between 10 and 16 volts before proceeding.
- With the scan tool, read active DTCs.
Is this DTC active?
Yes
- Go to step 2
No
- Refer to the Stored Lost Communication test procedure. Refer to Non-DTC Diagnostics/Communication - Diagnosis and Testing
- With the scan tool, read active DTCs.
- CHECK FOR ANY OF THE FOLLOWING ACTIVE DTC'S
- With the scan tool, read active DTCs from all modules.NOTE: Check for FCM configuration, CAN B or C hardware electrical, VIN Missing/Mismatch, battery or ignition related DTCs.
Does the scan tool display any active DTCs to the conditions listed above?
Yes
- Go to DIAGNOSTIC CODE INDEX table and perform the appropriate diagnostic procedure.
No
- Go to step 3
- With the scan tool, read active DTCs from all modules.
- VERIFY THAT THE TM IS ACTIVE ON THE BUS
- Turn the ignition on.
- With the scan tool, select Network View and select Advanced.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.
- Verify that the TM is active on the bus.
Is the TM active on the bus?
Yes
- Replace and program the Front Control Module in accordance with the Service Information.
- Perform the BODY VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Front Control (FCM) - Standard Procedure
No
- Go to step 4
- (A922) (A907) FUSED B(+) CIRCUITS OPEN
- Turn the ignition off.
- Disconnect the Trailer Lighting Module C2 harness connector.
- Using a 12-volt test light connected to ground, check the (A922) and (A907) Fused B(+) circuits.
Does the test light illuminate brightly on each circuit?
Yes
- Go to step 5
No
- Repair the (A922) or (A907) Fused B(+) circuit for an open or short.
- Perform the BODY VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Front Control (FCM) - Standard Procedure
- (F941) FUSED IGNITION SWITCH OUTPUT (RUN-START) CIRCUIT OPEN
- Turn the ignition on.
- Disconnect the Trailer Lighting Module C4 harness connector.
- Using a 12-volt test light connected to ground, check the (F941) Fused Ignition Switch Output circuit.
Does the test light illuminate brightly?
Yes
- Go to step 6
No
- Repair the (F941) Fused Ignition Switch Output circuit for an open or short.
- Perform the BODY VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Front Control (FCM) - Standard Procedure
- (Z923) GROUND CIRCUIT OPEN
- Using a 12-volt test light connected to 12-volts, check the (Z923) ground circuit.
Does the test light illuminate brightly?
Yes
- Go to step 7
No
- Repair the (Z923) ground circuit for an open.
- Perform the BODY VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Front Control (FCM) - Standard Procedure
- Using a 12-volt test light connected to 12-volts, check the (Z923) ground circuit.
- (D55) AND (D54) CAN B BUS CIRCUITS OPEN
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.