U007F: Control Module Communication Bus M Off

What it means

The control module communication bus M 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 M carries communication between the modules attached to that network. The control module monitors bus M traffic; this code sets when the bus is off, often after repeated errors put a CAN controller into its 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. Record all codes and freeze-frame data, then determine which bus M modules are unresponsive and whether a controller reports bus-off or repeated errors.
2. Inspect bus M wiring, connectors, and grounds for breaks, corrosion, loose pins, or shorts that may corrupt network traffic.
3. Review gateway status for bus M routing errors or isolation and compare it with the modules that are offline.
4. With power disconnected as the manufacturer specifies, test bus M wiring for continuity and shorts and check termination against the manufacturer's specification.
5. If wiring and termination are sound, isolate bus M modules using the manufacturer's procedure to find the controller or gateway disrupting communication.

Common fixes

• Repair damaged, broken, or shorted bus M wiring.
• Clean, secure, or replace corroded bus M connectors as needed.
• Replace a terminating resistor if it is outside the manufacturer's specification.
• Repair or replace the gateway if it routes or isolates bus M incorrectly.
• Repair or replace a module confirmed to be forcing bus M offline.

Can I keep driving?

Bus M being off can make connected systems unavailable or trigger limp mode, so keep driving limited and avoid relying on affected functions. Stop if a safety-related feature or essential control is impaired; continued use can leave more modules without needed information.