U0030: Vehicle Communication Bus A (+) Open
What to do
Expect a group of systems to have stopped working together, and possibly a car that will not start.
One broken wire does this. The two wires carrying a network are not a main and a spare — they carry the same message twice in opposite directions, and the computers read the difference — so breaking either one stops the whole network. Every computer then reports it cannot hear the others, which is why the fault list is alarming.
Do not let computers be replaced on the strength of that list.
One thing worth asking about: has the garage established which of the car's networks this actually is? This one is identified only by a letter, and the familiar test at the diagnostic socket checks a different network. A clean result there proves nothing here, and that is a common way to waste an afternoon.
And when the break is found, ask for the second wire to be inspected along the same stretch. They run twisted together, so whatever damaged one was touching the other — and a wire that is nearly through will finish the job later.
Mention accident repair, accessory installations, and any signs of rodents.
What this code means
The positive line of vehicle communication bus A is open circuit. See U0003 for what an open does to a differential pair and for the resistance test, and U0028 for why this bus must be identified before that test is applied.
Those two pages together give the whole procedure, and the order matters. Identify the network from the vehicle's topology, obtain its own resistance figure, then measure it at a point that is actually on it. A reading taken at the diagnostic socket may be measuring a different bus entirely, and a healthy result there has condemned many a good afternoon.
Where the break is follows the general list: connectors and splices, harness sections that flex, rodent damage, and loom cut and rejoined during accident repair.
And U0006's instruction applies wherever a break is found — inspect the partner conductor along the same length, because whatever damaged one was touching the other.
Likely causes
Start here
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Damaged wiring harness · moderate
- Failed bus terminating resistor · specialist
Less common
- Water ingress into connector · 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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