U0008: High Speed CAN Communication Bus (-) High
Do not keep driving
This is one of the two faults in this family that can damage things rather than just stopping them. Everything at U0005 applies.
The network wires are designed for a signal of two or three volts, and full battery voltage has got onto one of them — with every computer on the network connected to that same wire.
So be prepared for a repair in two stages. The damaged wire gets found and fixed, and the car may still not work, because something was damaged while the voltage was present. A garage that reports further faults after repairing the wiring is not moving the goalposts.
Two questions will very likely find the cause. Has anything been fitted to the car — alarm, tracker, dashcam, light bar, tow bar, stereo? And has the battery ever been connected the wrong way round, or jump-started backwards? People remember that and rarely mention it.
One thing specific to this wire: ask them to look at the earth connections of anything added, not just the live feeds. This particular wire runs nearest the earth wiring, and a bad earth on an accessory reaches it first — and the earth is the half installers treat as unimportant.
The car will need recovering.
What this code means
The negative line of the high speed data bus is being held high. Everything at U0005 applies unchanged, including the warning that separates this pair from the rest of the family: this fault can damage modules rather than merely stopping them.
Battery voltage on a bus wire is far outside what a transceiver is built for, and it reaches every module on the pair at once. So expect the repair to have two stages — the wiring fault found and fixed, and then whatever was damaged while the voltage was present. That is the nature of the fault rather than a moving quote.
One thing is specific to the negative line and worth checking first. CAN-L sits closer to earth in normal operation, so it is frequently routed nearer to earth wires and body grounds in a loom. A live feed that has found its way onto an earth point beside the pair, or a corroded earth stud carrying voltage it should not, reaches this wire before it reaches the other.
So where an aftermarket installation is suspected, its earth connection is worth as much attention as its live one — and that is the half installers treat as unimportant.
The two questions from U0005 remain the productive ones: what was fitted to the vehicle, and has the battery ever been connected the wrong way round.
Likely causes
Start here
- Wiring shorted to voltage · moderate
- Poor wiring splice in an aftermarket installation · moderate
- Corroded earth connection · moderate
- Damaged data bus wiring · moderate
Less common
- Faulty aftermarket alarm or immobiliser installation · moderate
- Damaged wiring harness · 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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