U0073: Control Module Communication Bus A Off
What to do
Nothing has broken. One of your car's computers has deliberately disconnected itself from the network, and it did so to protect everything else.
That sounds alarming and it is actually the system working as designed. Every computer counts its own failed transmissions, and when the count gets too high it is required to stop talking altogether — because a computer that keeps failing to transmit clogs the network for everyone else.
The important part: the computer that withdrew is not necessarily the faulty one.
It stopped because its messages kept failing — and messages fail when the wiring is bad. A damaged wire or a bad connection will drive a perfectly healthy computer into this state, and that computer's name is the one that ends up in the fault list. So do not let it be replaced on the strength of this code.
Two things you may have noticed, which are worth reporting because they confirm what is happening.
Does it work again after you switch the car off and on, and then fail a few minutes later? That is the computer rejoining the network and being forced out again — not an intermittent fault, and not something that has been fixed.
Does everything fail for a few seconds and then come right on its own? That is the network recovering the moment the struggling computer withdraws — and it is the clearest signature this fault has.
What this code means
A module has taken itself off communication bus A. This is a different kind of code from the rest of this range, and the difference is worth the whole page: nothing has broken, and a module has withdrawn deliberately.
Every node on a CAN network keeps a count of its own errors. Each failed transmission increases it; each successful one reduces it. When the transmit error count passes a defined threshold, the node is required to stop transmitting altogether — a state the protocol calls bus off.
That is protection rather than failure, and the reasoning is worth following. A node that keeps failing to transmit is, by that very fact, occupying the bus with attempts and retries. One faulty transmitter can prevent an entire healthy network from working, so the rules require it to withdraw before it does. The module removed itself so that everything else could keep running.
Which gives the reframing this page exists for: the module that went bus off is not necessarily the faulty one.
It withdrew because its transmissions kept failing — and transmissions fail when the bus is bad. A short, an open, a missing terminator or another node babbling will drive a perfectly healthy module into bus off, and it will be the one whose name appears in the fault memory. So a bus off code stored alongside a wiring code is one fault, and the wiring is it.
Two behaviours follow that explain things owners report and nobody believes.
Recovery differs between designs. Some modules rejoin automatically after the bus has been quiet for a defined period; some stay off until the ignition is cycled; some latch until the code is cleared. So a vehicle that works after being switched off and on again, and fails a few minutes later, is describing a module cycling through bus off recovery — not an intermittent fault, and not a repair.
And the network can appear to heal itself. If the withdrawn module was the one disrupting the bus, everything else recovers the instant it leaves. A vehicle that fails for several seconds and then comes right, repeatedly, is that exact sequence — and it is the single most recognisable signature in this range.
Likely causes
Start here
- Damaged data bus wiring · moderate
- Another module holding the bus down · specialist
- Corroded or loose connector · easy
- Failed bus terminating resistor · specialist
Less common
- Low system voltage · moderate
- Poor wiring splice in an aftermarket installation · 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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