U0002: High Speed CAN Communication Bus Performance
What to do
Expect things to be unreliable rather than broken: warning lights that appear and clear, an occasional stall or refusal to start, features that work most of the time. It will usually drive, but be aware it can cut out without warning, so treat a long or fast journey with caution until it is fixed.
What you have noticed is the most valuable thing in this diagnosis, because a workshop cannot reproduce it. Write down when it happens — hot or cold, wet or dry, rough road or smooth, just started or after an hour — and hand that over. A fault that tracks the weather points at water in a connector; one that tracks bumps points at a broken wire; one that only appears when everything is switched on points somewhere else again.
Mention anything that has been wired into the car after it was built: a tracker, an alarm, a dashcam, parking sensors, a phone kit, a towbar electrics kit. These commonly tap into the very wires this code is about, and a joint made by clamping onto the insulation rather than soldering is one of the most frequent causes. A garage has no way of knowing such a thing is fitted unless you say so.
What this code means
The high speed bus is working and not working well. Messages are arriving corrupted, being retransmitted, or dropping out and returning. What the bus is and how it is arranged is at U0001.
A network that is degraded rather than broken is harder to find than one that has failed outright, and the reason is that everything works most of the time. Modules retry failed messages, so a bus with a real fault on it can carry on delivering almost everything — until load rises, or a connection moves, and something important arrives late or not at all. The symptoms are correspondingly intermittent: warning lights that come and go, a stall that happens once a fortnight, a car that fails to start once and then starts perfectly for a week.
The causes are the ones that produce a marginal connection rather than an absent one.
- A partially broken conductor, or a connector with just enough corrosion to matter. Both are sensitive to temperature and vibration, which is why the fault tracks the weather or the road surface.
- A missing or wrong terminating resistor, which leaves signal reflections on the pair. This is a common consequence of a module having been removed and not properly accounted for.
- A module with a failing transceiver that garbles occasionally rather than falling silent — the worst case, because the protective bus-off mechanism at U0001 never triggers.
- An aftermarket device spliced into the bus. Trackers, alarms, dashcams, parking sensors and phone kits all commonly tap these two wires, and a joint made with a squeeze-on connector that pierces the insulation is a textbook cause — it works when new, corrodes at the piercing, and degrades a network that was designed on the assumption nobody would do that.
Because the fault is intermittent, what the driver has noticed is genuinely the best evidence available. A workshop cannot reproduce a once-a-fortnight event on a ramp.
Likely causes
Start here
- Poor wiring splice in an aftermarket installation · moderate
- Intermittent wiring fault · moderate
- Corroded or loose connector · easy
- Water ingress into connector · moderate
Less common
- Failed bus terminating resistor · specialist
- Another module holding the bus down · specialist
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.
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