U0035: Vehicle Communication Bus A (-) 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 connected to every computer on that network — so be prepared for a repair in two stages, as at U0005.
One thing you may be able to tell the garage is worth more than any test.
Does the fault appear only when something power-hungry is switched on? The heated rear screen, the stereo at volume, a heated seat, headlights.
If so, say exactly that. It sounds like nonsense and is a precise description of a bad earth connection shared with the network wiring — cheap to fix, and very hard to find without the clue.
Those bad earths usually come from something fitted after the car was built. Mention anything added, and ask for its earth connection to be checked as well as its power feed — the earth is the half installers treat as unimportant.
And ask for the car's network map to be read before the wiring is repaired, so there is a record of which computers were exposed to the voltage.
What this code means
The negative line of vehicle communication bus A is being held high. Everything at U0032 applies, including the module-damage warning from U0005 and the argument for reading the vehicle's topology before the repair rather than after.
U0008's point belongs here: this conductor runs nearest the earth wiring in a loom, so a bad earth on something added to the vehicle reaches it before it reaches the other wire.
Which gives the load-dependent signature. A fault that appears only when something heavy is switched on is a shared bad earth floating up under load, and it is a cheap fix that is very hard to find without that observation.
So ask for accessory earth points to be inspected as well as their live feeds — the earth is the half installers treat as unimportant, and it is the one that reaches this conductor.
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 aftermarket alarm or immobiliser installation · moderate
- Intermittent wiring fault · 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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