U0028 High Generic / SAE

U0028: Vehicle Communication Bus A

Network and Communication Bus Signal and Bus Off Emissions related

What to do

Expect a set of systems to have stopped working together, and possibly a car that will not start. Which systems depends on what this particular network carries, and that varies between manufacturers.

That is the honest first point of this page. Some of your car's networks have descriptive names, and this one has a letter. The letter is the manufacturer's own label — it does not tell you whether this is the fast network, the body network, or something else entirely.

So the first job is identifying which network it is, and that is a five-minute step with the right diagnostic equipment. Most manufacturer-level tools draw a map of the car showing which computers sit on which network.

It is worth asking whether that has been done, because skipping it is what makes these faults expensive. The familiar test at the diagnostic socket only checks one of the car's networks — and a perfect result there proves nothing about a different one.

Otherwise the usual advice applies: a long list of computers reporting that they cannot hear each other is one wiring fault, not a dozen failures, so do not let modules be replaced on the strength of it. Mention any accident repair, accessory installation or signs of rodents.

What this code means

A fault has been detected on vehicle communication bus A. See U0003 for the electrical faults that repeat on every bus in this range. This page covers the letters, which run from A here to F at U0080.

The first thing to establish is that the letter tells you nothing.

U0001, U0010 and U0019 name their networks by what they are — high, medium and low speed CAN — so a technician knows the medium and the wiring before opening a diagram. A lettered bus carries no such information. A is not necessarily the fastest, the powertrain one, or the first one fitted; it is whichever network this manufacturer designated A. On some vehicles it is not even copper — see U0046.

So the step that comes before any measurement is identifying which physical network this is, and there are three ways, in increasing order of usefulness:

The wiring diagram, which is definitive and slow.
The gateway's own routing configuration, which lists what it bridges.
The vehicle's topology as a scan tool draws it — most manufacturer-level tools display which modules sit on which bus, and that single screen turns a lettered code into a named place to look.

Skipping that step is what makes these codes expensive. Measuring pins 6 and 14 of the diagnostic socket tests the high speed bus — which may be perfectly healthy while bus A, on entirely different pins or none at all, is the one at fault. A clean 60 ohm reading proves nothing about a bus you have not identified.

Once identified, the rest of the range applies unchanged: the seven electrical faults at U0030–U0036, the error-rate argument at U0029, and the whole-network consequences at U0100.

Likely causes

Start here

Damaged data bus wiring · moderate
Corroded or loose connector · easy
Another module holding the bus down · specialist
Damaged wiring harness · moderate

Less common

Faulty gateway module · specialist
Poor wiring splice in an aftermarket installation · 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.