U007E: Control Module Communication Bus L Off

What it means

The control module communication bus L 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 L is the communication network for the modules connected to it. The control module monitors valid traffic on bus L; this code sets when bus L is off, commonly after repeated errors cause its 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. Capture all codes and freeze-frame data, then identify which bus L modules are offline and whether the CAN controller reports repeated errors or bus-off.
2. Inspect bus L wiring and connectors for damage, corrosion, poor pin fit, loose grounds, or shorts that could interrupt messages.
3. Check the gateway's bus L routing and isolation status and compare it with which modules fail to respond.
4. With the network powered down according to the manufacturer, test bus L wiring for continuity and shorts and verify termination against the manufacturer's specification.
5. If the wiring and termination pass, isolate bus L modules using the manufacturer's procedure to find a controller or gateway that is disrupting traffic.

Common fixes

• Repair damaged or shorted bus L wiring and connectors.
• Secure or repair corroded connections on bus L and at the gateway.
• Replace a terminating resistor if it fails the manufacturer's specification.
• Repair or replace the gateway if it incorrectly routes or isolates bus L.
• Repair or replace a module confirmed to be taking bus L offline.

Can I keep driving?

With bus L off, affected functions may disappear or enter limp mode; avoid high-speed or long trips and do not rely on systems that have stopped responding. Stop if a safety-related function is affected, since continued driving can leave modules without data they need.