U0075: Control Module Communication Bus C Off
What to do
Nothing has broken. One or more of your car's computers have deliberately stopped talking to protect the network — see U0073.
One question changes everything about this diagnosis, and it is free to answer: how many computers withdrew?
If it is one, the problem is most likely in that computer's own wiring, plugs or power supply.
If it is several, the problem is the network itself — and that is a much more useful thing to know than it sounds. Several computers reaching that state at the same time have all been failing to send messages on the same wiring, which says something about the wiring and nothing about any of them.
This is the situation where the fault list is most misleading. It now names several expensive computers, and none of them is faulty. Do not let any of them be replaced on that evidence — a workshop can spend a great deal before anything improves.
Ask instead for the network wiring itself to be measured, since that is the thing all of them have in common.
What this code means
A module has taken itself off communication bus C. See U0073 for what bus off means and U0074 for identifying which module withdrew.
One pattern is decisive and worth stating clearly: several modules going bus off at once is the network, not the modules.
Bus off is reached by accumulating transmit errors, and a module accumulates them because its transmissions are failing. Two or three modules reaching that threshold in the same period have all been failing to transmit on the same bus — which is a statement about the bus and not about any of them.
So the count changes the diagnosis completely. One module off points at that module's own branch — its wiring, its connectors, its supply. Several off points at the backbone, the terminators, or a node disrupting everybody.
And it is the case where U0073's warning matters most, because the fault memory now names several expensive modules and none of them is at fault. A workshop replacing modules on that evidence can spend a great deal before anything improves.
Where several are off, the U0003 measurements are the right next step — resistance across the pair for this network, then each conductor against earth — because they test the thing all of them have in common.
Likely causes
Start here
- Damaged data bus wiring · moderate
- Failed bus terminating resistor · specialist
- Another module holding the bus down · specialist
- Corroded or loose connector · easy
Less common
- Poor wiring splice in an aftermarket installation · moderate
- 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
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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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