U0078 High Generic / SAE

U0078: Control Module Communication Bus F Off

Network and Communication Bus Signal and Bus Off Emissions related

What to do

If this has been "fixed" before and keeps coming back, read this — because it may never have been fixed at all.

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

Here is the trap. On many cars, that computer rejoins the moment the fault codes are cleared. So the warning lights go out, everything works, and the car drives away.

That looks exactly like a repair and it is not one. The wiring fault is untouched, and the computer will withdraw again as soon as enough errors build up — which might be minutes or might be weeks.

So the question worth asking is not what was done, but what was found. "We cleared the codes and it was fine" is not a diagnosis, and a fault that has come back three times has probably never actually been investigated.

Ask what was measured, and what the result was.

One thing that genuinely helps: ask for the codes to be read and recorded before they are cleared, and then cleared once deliberately. That way the next occurrence is a clean record rather than a mixture of old and new faults, which makes it far easier to find.

What this code means

A module has taken itself off communication bus F. See U0073 for what bus off means and U0080 for what a sixth lettered network usually implies about the vehicle.

One point belongs here and it is about what a repair actually is: clearing the code is not one.

U0073 explains that recovery differs between designs — some modules rejoin automatically, some after an ignition cycle, some only when the fault memory is cleared. So on a good many vehicles, clearing the codes brings everything back to life.

And that is genuinely misleading, because it looks exactly like a repair. The module rejoins, the warning lamps go out, the vehicle drives away, and the underlying wiring fault is untouched. It returns as soon as enough errors accumulate again — which may be minutes or weeks depending on how marginal the fault is.

Which produces a pattern worth naming: a vehicle that has been "fixed" several times by having its codes cleared. Each visit ended with everything working. None of them changed anything.

So the instruction is to establish what was found rather than what was done. "The codes were cleared and it drove away fine" is not a diagnosis, and a fault that has come back three times has never actually been looked at.

And it is worth clearing the codes deliberately once, after reading them, precisely so that the next occurrence is a fresh record rather than a mixture of old and new — see U0064.

Likely causes

Start here

Damaged data bus wiring · moderate
Corroded or loose connector · easy
Intermittent wiring fault · moderate
Poor wiring splice in an aftermarket installation · moderate

Less common

Another module holding the bus down · specialist
Failed bus terminating resistor · specialist

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.