U0077: Control Module Communication Bus E Off

What it means

A control module has detected that communication bus E is off. The code does not specify which module or the underlying cause.

At a glance

• Usually lights the warning light of the system that lost communication

How it works

Bus E carries messages between the modules connected to that network. The control module monitors whether valid communication is available on bus E; this code sets when that bus is detected as off, but does not identify the failed module or cause.

Common causes

Most likely first.

1. Persistent wiring fault on bus B
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 check which modules on bus E respond and whether other bus-off or communication codes are present.
2. Check the vehicle information for the bus E route and inspect its wiring and connectors for damage, looseness, corrosion, or signs of a short.
3. Check whether the gateway is reporting bus E errors or routing faults, and compare its status with the modules that have stopped responding.
4. With the network inspected and powered down as specified by the manufacturer, test bus E wiring for continuity and shorts, and check termination against the manufacturer's specification.
5. If the wiring and termination pass, isolate modules on bus E according to the manufacturer's diagnostic procedure to find a device that is disrupting communication.

Common fixes

• Repair damaged or corroded bus E wiring or connectors.
• Correct a poor connection at a bus E connector or gateway.
• Repair or replace the gateway if testing confirms it routes bus E messages incorrectly.
• Replace a faulty bus E terminating resistor if it fails the manufacturer's specification.
• Repair or replace a module that testing shows is taking bus E offline.

Can I keep driving?

Avoid relying on functions that have stopped working, since multiple modules may lose data or enter reduced operation. If steering, braking, or other safety-related functions are affected, stop driving and arrange recovery; otherwise limit driving until bus E communication is restored.