U0077 High Generic / SAE

U0077: Control Module Communication Bus E Off

Network and Communication Bus Signal and Bus Off Emissions related

What to do

If the fault happens only when something particular switches on, that is the answer rather than a coincidence — and only you are likely to notice it.

One of your car's computers has deliberately stopped talking to the network — see U0073.

These withdrawals happen when errors build up, and several things on a car cause bursts of electrical interference when they run. A radiator fan starting. The heated rear screen. The air conditioning compressor. The power steering working hard on full lock.

So think about what the car was doing.

Only when the cooling fan comes on?
Only on full steering lock?
Only with the heated rear screen on?

Any of those names the circuit that is interfering with the network — usually because that circuit's earth connection has gone bad, or because its wiring has ended up lying against the network wires after a repair.

That turns a vague intermittent fault into a short, specific search. A garage cannot easily reproduce "only when the fan runs"; you already know it. Write it down and hand it over — it is worth more than an hour of their time.

What this code means

A module has taken itself off communication bus E. See U0073 for what bus off means and U0074 for identifying which module withdrew.

One version of this fault carries its own answer, and recognising it saves the whole search: a module that goes bus off only when a particular system operates.

Bus off requires errors to accumulate, so anything that produces bursts of interference or voltage disturbance can drive a marginal node over the threshold. And several systems on a vehicle do exactly that when they run — a high-current motor starting, a heater element switching, an ignition system, a compressor clutch engaging, an electric power steering motor working hard.

So a vehicle that loses its network only when the radiator fan starts, or only on full steering lock, or only with the rear screen heater on, is not describing a coincidence. It is naming the circuit that is coupling into the bus — usually because that circuit's earth is poor, or because its wiring has come to lie against the bus pair after a repair.

Which turns an intermittent network fault into a specific and short investigation, and the observation is the owner's rather than the workshop's. A technician cannot easily reproduce "only when the fan runs"; a driver already knows it.

So the useful instruction is to notice the trigger and report it, and to expect the answer to be an earth or a routing problem on the triggering circuit rather than a fault on the network at all.

Likely causes

Start here

Corroded earth connection · moderate
Damaged data bus wiring · moderate
Intermittent wiring fault · moderate
Poor wiring splice in an aftermarket installation · moderate

Less common

Another module holding the bus down · specialist
Low system voltage · moderate

Symptoms

Components involved

Information, not instruction

CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.

Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.

Community experience

Be the first to write this up

Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.

What makes one worth reading:

The car — make, model, year and engine. The same fault behaves differently on each.
What you measured, not only what you replaced.
What actually fixed it — or that nothing did, which is worth knowing too.