U0028–U0064 Vehicle Communication Bus A - general fault

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

U0028: Vehicle Communication Bus A - general fault.

U0028 is a network code (communication between control modules), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

1. Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
2. Check the connector at the part for corrosion, and that it is fully seated.
3. Clear the code and see whether it returns, and under what conditions.

What actually causes this on a VAG

This is the general version of a data network fault. One of the car's control units reported a problem with a shared CAN bus - the wiring the modules use to talk to each other - without being able to say anything more specific about what went wrong. On its own it tells you where to look, not what is broken. Two things make it more useful than it first appears. First, VAG cars do not have one network, they have several - powertrain, comfort, infotainment and others - joined together by a central gateway, and 'A' is simply the reporting module's own label for whichever bus it sits on. So the first question is always which module logged it, because that is what identifies the bus. Second, it has close relatives that appear in the same situations and are diagnosed the same way: U0029 is the same bus reported as implausible or under-performing rather than failed, and U0030 is that bus with a wiring short. And it very often turns up with a crowd of other communication codes - that crowd is itself a clue.

Most likely causes, in order

1. A flat, weak or failing battery. By far the most frequent cause of a screenful of bus codes on VAG vehicles. Low or unstable voltage makes modules drop offline for a moment and everything logs a complaint about everything else. Test the battery under load and check the charging system, then clear the memory and re-scan. A great many of these faults simply never come back.
2. A module that has lost its power or earth. If one control unit loses a fuse, a supply or a ground, it goes silent and its neighbours all report a bus problem. This is why a full vehicle scan matters: what you are looking for is which modules answer and which are missing from the list entirely. The one that is silent is reporting nothing at all, so it will never appear as a fault code - it appears as a gap. Then it is a matter of checking that module's fuses, supply and connectors.
3. Damaged bus wiring or a corroded connector. CAN runs as a twisted pair and it is intolerant of chafe, water and bodged joins. Look wherever the loom flexes or gets wet - door boots, boot lid, under the seats, the scuttle area - and especially if the car has damp carpets or has had accident repair. Fix the water ingress before the wiring, or you will be back.
4. Aftermarket electrics tapped into the network. Trackers, alarms, dashcams, tow bar modules, parking sensors and non-original head units are routinely spliced into CAN wiring and are a common source of general bus faults. If something was fitted shortly before this started, disconnect it and re-test - that costs nothing and it is the fastest way to rule it in or out.

Worth knowing: Charge and properly test the battery, then clear the fault memory and drive the car before paying anyone to chase wiring. Historic bus codes stored during a flat battery, a jump start, or a spell of the car standing are extremely common on these vehicles, and treating them as live faults wastes real money on a car that has already fixed itself. When you do pay for diagnosis, insist on a full scan of every module rather than a generic OBD code reader - on a bus fault the useful information is which modules are present, which have dropped off the list, and what each one is complaining about, and a plug-in reader will never show you that.

Common questions

What does U0028 mean?

U0028 means Vehicle Communication Bus A - general fault. The ECU sees a problem in this circuit but cannot narrow it to a specific short or open. Start at the connector and the wiring before condemning the part.

Can I drive with U0028?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

Related reading

• Several warning lights on at once: usually one cause, not many
• Do you actually need a VAG-specific scan tool?
• Key not recognised or car won't unlock: VW and Audi
• Why the same fault code has completely different causes