U0074: Control Module Communication Bus B Off
What to do
Nothing has broken. One of your car's computers has deliberately stopped talking to the network to protect the rest — see U0073.
One thing is worth asking for, and it costs nothing: which computer actually withdrew?
This code names the network, not the culprit. But the information is already in the car — the computer that withdrew recorded it, and everything else will be reporting that it has lost contact with one particular unit.
So whichever computer everything else has lost touch with is the one that stepped back, and its own wiring, plugs, power supply and earth are where to look first.
Starting there is not the same as blaming it. Its own branch of the wiring is simply the place where a fault would most easily force it out — so that is where to inspect, and it is not a reason to buy the part.
Mention any accident repair, accessory installation or signs of rodents.
What this code means
A module has taken itself off communication bus B. See U0073 for what bus off means and why the withdrawn module is not necessarily the faulty one.
One practical point belongs here: the code names the bus, and the module is recorded separately.
"Bus B off" says which network was abandoned. It does not say by whom — and that is the information the diagnosis actually needs. It is held in the module that stored the code, since a module reports its own withdrawal, and in the codes stored by everything else, which will be reporting lost communication with one particular module.
So the identification is free and it is a matter of reading rather than testing. Whichever module every other module has lost contact with is the one that went off, and that is where the wiring investigation starts — its own branch of the network, its connectors, its supply and its earth.
And U0073's warning applies to what happens next: starting there does not mean suspecting it. A module's own branch is where a fault would most easily drive it into bus off, which is a reason to inspect that wiring first and not a reason to order the part.
Likely causes
Start here
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Another module holding the bus down · specialist
- Failed power or earth supply to the module · moderate
Less common
- Failed bus terminating resistor · specialist
- Poor wiring splice in an aftermarket installation · moderate
Symptoms
Components involved
Information, not instruction
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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
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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.
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