U0033: Vehicle Communication Bus A (-) Open
What to do
Expect a group of systems to have stopped working together. One broken wire does this — see U0030 for why breaking either wire stops the whole network.
Two things are worth asking for.
Has the garage established which of the car's networks this actually is? It is identified only by a letter, and the familiar test at the diagnostic socket checks a different network, so a clean result there proves nothing here.
And when the break is found, ask for the second wire to be inspected along the same stretch — and for a damaged length of loom to be replaced rather than one wire spliced.
There is a real reason for that beyond thoroughness. The pair works because the two wires are electrically identical — same length, same route, same twist. Repairing one and not the other makes them slightly different, and on a marginal network that is enough to turn a fixed fault into an intermittent one that is far harder to find.
Mention accident repair, accessory installations, and any signs of rodents.
What this code means
The negative line of vehicle communication bus A is open circuit. Everything at U0030 applies — identify the network before measuring it, and use that network's own resistance figure rather than the familiar one.
U0006's instruction is the one to carry from here: the partner conductor runs in the same loom and was in contact with whatever broke this one. Inspect it along the same length, and where a section of harness is damaged, replace the section rather than splicing a single wire.
That last point has a reason beyond thoroughness. A bus pair works because the two conductors are electrically alike — same length, same routing, same twist. A repair in one and not the other introduces a small asymmetry, and asymmetry is precisely what the twisting exists to prevent. On a marginal network it is enough to turn a repaired fault into an intermittent one.
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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