U0044: Vehicle Communication Bus B (-) High
What to do
This is one of the faults that can damage things rather than just stopping them. Battery voltage has got onto a network wire. Be prepared for a repair in two stages, as at U0005.
Two things worth telling the garage.
Does the fault appear only when something power-hungry is switched on? The heated rear screen, the stereo at volume, headlights. If so, say exactly that — it is a precise description of a bad earth shared with the network wiring, and it is cheap to fix and hard to find without the clue.
And has anything been connected to the car's network after it was built? A tracker, insurance box, dashcam or fleet device. Those installations run a power feed alongside the network wires, which is exactly the condition this fault needs.
One thing that may save confusion later. If faults appear on a second, apparently unrelated network too, that is not necessarily a second problem. Damage on one side of the car's internal translator can leave it misbehaving on the other. Ask for all the stored codes to be considered together rather than worked through one at a time.
What this code means
The negative line of vehicle communication bus B is being held high. Everything at U0041 applies, including the module-damage warning from U0005 and the reason an added device makes this fault likelier.
And U0008's point holds: this conductor runs nearest the earth wiring, so a bad earth on an accessory installation reaches it before the other wire — giving the load-dependent signature at U0035, where the fault appears only when something heavy is switched on.
One thing is worth adding for a vehicle with several networks, which is what a lettered bus implies. A fault of this kind does not always stay on one network. Where a gateway bridges two buses, damage to a transceiver on one side can leave the gateway itself misbehaving on the other — so codes appearing on a second, apparently unrelated bus are not necessarily a second fault.
Which is an argument for reading all the stored codes together rather than working through them individually, and for suspecting one event rather than several.
Likely causes
Start here
- Corroded earth connection · moderate
- Wiring shorted to voltage · moderate
- Poor wiring splice in an aftermarket installation · moderate
- Damaged data bus wiring · moderate
Less common
- Faulty gateway module · specialist
- Faulty aftermarket alarm or immobiliser 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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