U0042: Vehicle Communication Bus B (-) Open
What to do
Expect a group of systems to have stopped working together. One broken wire does this.
As at U0039: a normal resistance reading does not necessarily clear the wiring here, because where the break falls changes what the meter shows. That is a reason to know the layout before relying on the test, not a reason to distrust it.
And when the break is found, ask for the damaged length of loom to be replaced rather than one wire spliced. The two wires work because they are electrically identical — same length, same route, same twist — and repairing one and not the other makes them slightly different, which on a marginal network turns a fixed fault into an intermittent one.
One reassurance about cost. The garage will need the wiring diagram to identify this network at all — and that same diagram shows the route and the ends of the wiring. So it is not an extra charge; it is the step that makes everything after it meaningful.
Mention accident repair, accessory installations, and signs of rodents.
What this code means
The negative line of vehicle communication bus B is open circuit. Everything at U0039 applies — including that where the break falls relative to the two terminators decides what a resistance reading shows, so a normal value does not clear the wiring on a network whose layout is unknown.
And U0033's point about repairing the pair symmetrically belongs here: the two conductors work because they are electrically alike, so a splice in one and not the other introduces exactly the asymmetry the twisting exists to remove. Replace the damaged section rather than repairing a single wire.
One practical note for a lettered bus. Because the network has to be identified before it can be traced, the wiring diagram is needed anyway — and that same diagram gives the backbone's route and the position of its terminators. Obtaining it is not an extra step; it is the step that makes every subsequent measurement mean something.
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
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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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