U0017: Medium Speed CAN Communication Bus (-) High
What to do
This is one of the faults that can damage things rather than just stopping them — full battery voltage has got onto a network wire. Be prepared for the repair to have two stages, as at U0005.
One thing you may be able to tell the garage is worth more than any test they can do.
Does the fault appear only when something power-hungry is switched on? The heated rear screen, the stereo at volume, a heated seat, the powered tailgate.
If so, say exactly that. "It only does it with the heated rear screen on" sounds like nonsense and is a precise description of a bad earth connection shared with the network wiring — and it is a cheap fix that is very hard to find without that clue.
Where those bad earths come from is usually something fitted after the car was built. An installer needs an earth as well as a live, and the earth is the half people treat as unimportant — a self-tapping screw into a painted boot floor, or a bolt shared with a seat belt anchor. Mention anything that has been fitted, and ask for its earth connection to be checked as well as its power feed.
What this code means
The negative line of the medium speed data bus is being held high. See U0005 for the module-damage warning and U0008 for why the earth side of an accessory installation reaches this conductor first.
That last point is worth repeating on this bus because it is where accessories actually get earthed.
An installer needs an earth as well as a live, and the earth is the half treated as unimportant — a self-tapper into a boot floor, a bolt shared with a seat belt anchor or a seat rail, a body panel that is painted and only conducts because the screw bit through it.
A poor earth does not stay at zero volts. It floats up under load — and where it is bundled with the bus pair, that voltage appears on the conductor nearest it.
Which produces a fault worth recognising: one that appears only when something heavy is switched on. A rear screen heater, an amplifier at volume, a powered tailgate, a heated seat. "It only does it when the heated rear screen is on" sounds like nonsense and is an exact description of a shared bad earth, and a driver who has noticed that pattern has done the hardest part of the diagnosis.
So ask for accessory earth points to be inspected, not only their live feeds, and expect the U0005 possibility that a module was damaged.
Likely causes
Start here
- Corroded earth connection · moderate
- Poor wiring splice in an aftermarket installation · moderate
- Wiring shorted to voltage · 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
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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