U0056: Vehicle Communication Bus D Performance
What to do
Everything may still work, and this is worth attending to now — see U0011.
One observation from you is worth more than a workshop test drive here.
Does it only happen on the first start of the day? Or only after the car has been standing?
That is not a vague description — it is precise, and it points at something specific. The hardest thing a car's network does is wake up: the computers come alive at slightly different moments, and a network that is only just healthy copes perfectly once everything is running and makes mistakes exactly at that transition.
A garage that tests the car after it has been running for twenty minutes is measuring the easy part.
So write down when it happens and ask them to capture the network's error counts across a sleep and a wake rather than during a drive. That is a reasonable, specific request, and it is the difference between finding this and not.
And mention anything fitted to the car — a tracker, dashcam or insurance box — since anything added to a network is a well-known cause of exactly this.
What this code means
Vehicle communication bus D is working, and not working properly. See U0011 for what a performance code means and U0055 for the sleep argument.
Those combine into a specific and easily missed case: errors that occur only around sleep and wake.
The transitions are the hardest thing a network does. Modules power up at slightly different rates, transceivers stabilise at different moments, and the bus carries traffic while some participants are still arriving. A network that is marginal — a connector corroding, a terminator drifting, a stub too long — copes perfectly with steady traffic and produces errors at exactly those transitions.
Which gives a diagnostic instruction that is easy to state and rarely followed: capture the error counters across a sleep and a wake, not during a test drive. A vehicle that has been running for twenty minutes has passed the difficult part — and a workshop that measures while the engine is running is measuring the easy case.
And it explains a pattern owners report and nobody credits. "It only does it on the first start of the day", or "only after it has been standing". On this network that is not vagueness. It is the transition case, and it is worth writing down.
Otherwise U0011's causes apply, and U0037's long-stub argument is the one that produces errors on wiring that measures perfectly.
Likely causes
Start here
- Corroded or loose connector · easy
- Poor wiring splice in an aftermarket installation · moderate
- Failed bus terminating resistor · specialist
- Damaged data bus wiring · moderate
Less common
- Intermittent wiring fault · moderate
- Low system voltage · 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
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
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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