U0040: Vehicle Communication Bus B (+) Low
What to do
Expect a group of systems to have stopped working together. A network wire is being held down, so messages cannot get through.
Two possibilities, and they need different work.
A damaged wire — shorted to the bodywork somewhere. Mention anything fitted to the car (a tracker, dashcam, alarm, stereo or tow bar) since an added earth beside the network wires is a common cause, and say whether the fault came and went before becoming permanent, which points at a wire rubbing through.
Or a computer holding the network down by itself. A failed module can pull the line to earth exactly as a damaged wire does, and no amount of inspecting wiring will find it. The standard way to identify it is to unplug computers one at a time until the network recovers — slow, methodical, and much faster than tracing a wire that turns out to be undamaged.
So if the wiring has been checked and proven good, that is progress rather than a dead end. It points at the second possibility.
Do not let computers be replaced before that test has actually been done, though — a long fault list is not evidence against any particular one.
What this code means
The positive line of vehicle communication bus B is being held low. See U0004 for the short-to-earth argument and U0031 for why the voltage test works before the network has been identified.
One cause is worth raising ahead of the wiring on any bus where a module has been added or replaced: a module can hold a bus down by itself.
A transceiver that has failed with its output stuck low pulls the line to earth exactly as a chafed wire does, and no amount of inspecting the harness finds it. U0100 names this among its causes and it applies with force here.
The method that separates the two is disconnection rather than measurement. Unplug modules from the network one at a time and watch the line recover — the module that restores the bus when removed is the one holding it down. That is slow, it is the standard approach, and it is much faster than tracing a conductor that is not damaged.
So a bus that stays low with the harness proven good is a module, and the ordinary wiring causes should be eliminated first only because they are cheaper to check.
Likely causes
Start here
- Wiring shorted to ground · moderate
- Another module holding the bus down · specialist
- Damaged data bus wiring · moderate
- Poor wiring splice in an aftermarket installation · moderate
Less common
- Damaged wiring harness · moderate
- Water ingress into connector · 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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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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