U0026: Low Speed CAN Communication Bus (-) High
What to do
Everything probably still works, and as at U0023 this is one to deal with promptly rather than leave. The network has switched to its other wire and carried on — but battery voltage is still sitting on a wire connected to every computer on it.
One thing you may be able to tell the garage is worth more than any test they can run.
Does the fault appear only when something power-hungry is switched on? The heated rear screen, the stereo at volume, a heated seat, a powered tailgate.
If so, say exactly that. It sounds like nonsense and it is a precise description of a bad earth connection shared with the network wiring — a cheap fix that is very hard to find without the clue.
Those bad earths usually come from something fitted after the car was built. An installer needs an earth as well as a live, and the earth is the half treated as unimportant — a screw into a painted boot floor, or a bolt shared with a seat belt anchor. Mention anything fitted, and ask for its earth to be checked as well as its power feed.
What this code means
The negative line of the low speed data bus is being held high. Everything at U0023 applies, including the instruction that separates it from the rest of this family: have this one seen promptly even though nothing has stopped working, because the fallback hides the fault without removing the damaging voltage.
And U0008's point belongs here too: 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 produces the load-dependent signature from U0017. A fault that appears only when something heavy is switched on — a heated rear screen, an amplifier at volume, a heated seat — is a shared bad earth floating up under load, and a driver who has noticed that pattern has supplied the hardest part of the diagnosis.
So ask for accessory earth points to be inspected as well as their live feeds, and expect U0005's 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
- Intermittent wiring fault · moderate
- 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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