U007C: Control Module Communication Bus J Off

What it means

The control module communication bus J is off — the gateway has shut down or isolated this bus, often due to repeated bus errors or a "bus off" event in the CAN controller.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

Bus J connects the modules that exchange data on that network. The control module monitors valid bus J traffic; this code sets when bus J is off, often after repeated errors cause a CAN controller to enter a bus-off state.

Common causes

Most likely first.

1. CAN controller in bus-off state
2. Gateway module routing CAN messages incorrectly
3. CAN terminating resistor (120 Ω) drifted out of spec

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Save all codes and freeze-frame data, then determine which bus J modules are offline and whether the CAN controller has logged repeated errors or bus-off.
2. Inspect bus J wiring and connectors for abrasion, corrosion, loose pins, damaged grounds, or shorts that could corrupt messages.
3. Check the gateway's bus J status for routing errors or isolation and compare it with module response failures.
4. With the network powered down as the manufacturer directs, test bus J for continuity and shorts and verify termination against the manufacturer's specification.
5. If the wiring and termination are within specification, isolate bus J modules using the manufacturer's procedure to locate the controller or gateway disrupting the bus.

Common fixes

• Repair damaged or shorted bus J wiring and connectors.
• Restore clean, secure connections at bus J modules and the gateway.
• Replace a terminating resistor if it does not meet the manufacturer's specification.
• Repair or replace the gateway if it incorrectly routes or isolates bus J.
• Repair or replace the module confirmed to be driving bus J into bus-off.

Can I keep driving?

Since bus J being off can trigger limp mode or remove functions, keep speed and trip length low and avoid relying on affected systems. Stop driving if a safety-related function is impaired; continued use can leave connected modules without needed data.