U0004: High Speed CAN Communication Bus (+) Low
Do not keep driving
The car will probably not start, and the dashboard may be dead or lit with warnings from everything at once. That is one damaged wire, not a dozen failed computers.
The two wires that carry your car's internal network are being held down by a short to the bodywork, so no messages can get through.
Two things are worth telling the garage, because they save real time.
Has anything been fitted to the car — a tracker, dashcam, alarm, stereo, tow bar, insurance box? An added earth connection or a screw through the loom next to the bus wires is a common cause, and unplugging the device to re-test is far quicker than stripping a dashboard.
And did the fault come and go before it became permanent? That is typical of a wire rubbing through against the bodywork, and it tells a technician to move the loom by hand rather than to reach for a meter.
The car will need recovering. Do not let computers be replaced on the strength of the long list of faults — they are almost certainly all fine.
What this code means
The positive line of the high speed data bus is being held low. See U0003 for the pair, the resistance test and why one damaged wire stops the whole network.
This one has a voltage signature that identifies it in seconds, and it is worth knowing because it distinguishes a short from an open without disturbing anything.
At rest, both bus wires sit at about 2.5 volts. When a message is sent, CAN-H rises to roughly 3.5 and CAN-L falls to roughly 1.5 — symmetrically, which is the whole point. A CAN-H line sitting at zero and never moving is shorted to earth.
Where that happens is nearly always mechanical rather than electrical. A conductor chafed through against the body, a bracket, or an edge of a panel — which is why the fault frequently follows the loom's route rather than any component, and why it is often intermittent before it is permanent.
So the test that finds it is physical: move the loom. A bus that recovers when a section of harness is lifted or flexed has told you where to look, and that is faster than any meter on a fault that is currently present.
One thing to check before searching the vehicle's own wiring. An aftermarket installation that tapped an earth beside the bus pair, or ran a screw through the loom, is a common cause — and it is much quicker to unplug a tracker or a dashcam and re-test than to strip a dashboard.
Likely causes
Start here
- Wiring shorted to ground · moderate
- Damaged data bus wiring · moderate
- Poor wiring splice in an aftermarket installation · moderate
- Damaged wiring harness · moderate
Less common
- Water ingress into connector · moderate
- Another module holding the bus down · specialist
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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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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