U0071: Vehicle Communication Bus E (-) High
What to do
Do not leave this running while somebody waits for the fault to reappear — see U0068. Battery voltage on a network wire damages things each time it happens, so an intermittent version has done it many times over.
Two observations are worth reporting.
Does it appear only when something power-hungry is switched on? That is a bad earth shared with the network wiring.
And are there also faults about low battery voltage, or hard starting? If so, that is probably not a second problem. One badly corroded earth connection does both: under light load it lets voltage leak onto the wire beside it, and under heavy load it drags the whole car's voltage down. So codes of both kinds usually mean one bad connection rather than two faults, and chasing them separately sends people looking in two places for one answer.
Mention everything that has been fitted to the car, and ask for the earth connections to be checked as well as the power feeds.
What this code means
The negative line of vehicle communication bus E is being held high. Everything at U0068 applies, including that an intermittent short to power is cumulative and should not be left running while evidence is gathered.
And U0008's point holds: this conductor runs nearest the earth wiring, so a bad earth on an accessory reaches it first, giving the load-dependent signature at U0035.
One thing is worth adding, because it connects this fault to U0070's cause and they are frequently confused.
A poor earth produces both. Under light load it lets the conductor beside it rise — this fault. Under heavy load it drags the whole system's voltage down — U0070's. So a vehicle showing codes of both kinds is not describing two faults; it is describing one bad connection at two moments.
Which is a useful thing to recognise, because chasing them separately means looking for a short and a supply problem in different places when the answer is a single earth stud that has corroded behind a trim panel.
Likely causes
Start here
- Corroded earth connection · moderate
- Poor wiring splice in an aftermarket installation · moderate
- Wiring shorted to voltage · moderate
- Intermittent wiring fault · moderate
Less common
- Low system voltage · moderate
- Damaged data bus wiring · 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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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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