U0039: Vehicle Communication Bus B (+) Open
What to do
Expect a group of systems to have stopped working together. One broken wire does this — see U0030.
Two things worth asking for.
Has the garage established which of the car's networks this is? It is identified only by a letter, and the familiar test at the diagnostic socket checks a different one.
And be aware that a normal resistance reading does not necessarily clear the wiring here. The network has a terminator at each end, so where the break falls changes what the meter shows — a break past the last computer can read completely normal. That is not a reason to distrust the test; it is a reason to know the layout before relying on it.
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.
Mention accident repair, accessory installations, and any signs of rodents.
What this code means
The positive line of vehicle communication bus B is open circuit. See U0003 for what an open does to a differential pair, U0028 for identifying the network before measuring it, and U0030 for the procedure in order.
One thing follows from U0037's topology argument and is worth knowing when the resistance reads high.
The two terminators sit at the physical ends of the backbone. So where the break falls decides what the meter shows: a break in the middle leaves each half with one terminator and reads about double the normal value from either side, while a break beyond the last module still leaves both terminators connected and may read entirely normal.
Which means a correct resistance reading does not clear the wiring on a network whose shape is unknown. It narrows where the break can be, and that is genuinely useful — but only once the topology is known.
Otherwise the general list applies, and U0006's instruction to inspect the partner conductor wherever a break is found.
Likely causes
Start here
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Failed bus terminating resistor · specialist
- Damaged wiring harness · moderate
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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