U0012: Medium Speed CAN Communication Bus (+) Open
What to do
The car will almost certainly drive normally. Expect a group of electrical fittings to have stopped working together.
One free check comes before anything else, and it fixes a good share of these. Open and close each door — and the tailgate — slowly, and watch whether anything changes.
If it does, the problem is a broken wire inside the rubber concertina between the door and the body. That wiring is flexed every single time the door is opened, over tens of thousands of times, and it breaks inside insulation that still looks perfect from outside. It is a repair to a few centimetres of wire, and telling the garage what you noticed points them straight at it.
Otherwise mention any damp carpets, water in the boot, or work involving the boot trim, sill trims or a stereo installation.
Check your lights, indicators and brake lights before driving at night.
What this code means
The positive line of the medium speed data bus is open circuit. See U0003 for what an open does to a differential pair — the second wire is half the measurement, not a spare — and U0010 for what this network carries.
Where this bus breaks has a shorter list than the high speed one, because of where it goes. It runs to the doors, and a door harness passes through a flexible rubber boot that is flexed every single time the door is opened — tens of thousands of times. Copper work-hardens and breaks inside insulation that still looks perfect.
Which gives the free check that resolves a good proportion of these: does the fault change as a door or the tailgate is moved? A code that comes and goes, or body functions that work with a door at one angle and not another, is a broken conductor in that boot rather than anything else.
After that: the tailgate hinge, which fails the same way, and the connectors behind rear and sill trim that come off for boot liners, seat work and audio installations.
Likely causes
Start here
- Broken wire in the door harness boot · moderate
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Damaged wiring harness · moderate
Less common
- Failed bus terminating resistor · specialist
- 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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