U0046 Moderate Generic / SAE

U0046: Vehicle Communication Bus C

Network and Communication Bus Signal and Bus Off

What to do

Expect a group of systems to have stopped — most often the entertainment, navigation and phone systems, though that depends on what this particular network carries. The car will usually drive normally.

One thing on this page is worth knowing before a garage starts work, because it can save a wasted diagnosis.

Not every network in a car is a pair of wires. Many cars carry their entertainment systems on a ring of optical fibre — light rather than electricity. Every unit in that ring passes the signal on to the next, so one unit switched off, one dirty connector end, or one fibre bent too tightly stops the whole ring — and none of that can be found with an electrical meter.

So it is worth asking what kind of network this actually is before anyone tests it. Reaching for a multimeter on an optical ring finds nothing, whatever is wrong.

And one clue you may be able to give. If exactly one thing has stopped working rather than a group, that points at a newer style of network where each unit has its own separate connection — which is a much smaller repair.

If a stereo, amplifier or screen has been fitted or removed recently, say so. On an optical ring, a fibre kinked behind the dashboard during an installation is a common and completely invisible cause.

What this code means

A fault has been detected on vehicle communication bus C. See U0028 for why a lettered bus must be identified before anything is measured — and this page for the reason that warning is not merely bureaucratic.

A lettered bus makes no promise that it is copper at all, and applying the wrong test to the wrong medium is how a diagnosis goes badly wrong.

Three kinds of network turn up behind these letters.

A twisted pair carrying CAN, which is what the rest of this range assumes: two conductors, a differential signal, terminators at the ends, and the tests at U0003.

An optical ring, used for infotainment on a good many vehicles. A plastic optical fibre carries light from one module to the next in a loop, and every module is a repeater — the signal arrives, is regenerated, and passes on. So the whole ring depends on every module in it. One unit powered down, one connector with a fingerprint on its end face, or one fibre bent past its minimum radius takes the entire ring down, and nothing about that is visible or measurable with a meter. Resistance and voltage tests mean nothing here; the question is whether light is arriving.

And automotive Ethernet on newer vehicles, which inverts the range's central rule. Ethernet is switched and point-to-point rather than shared — each module has its own link to a switch — so a single bad link takes out one module instead of everything. A fault affecting exactly one module on a network is evidence about the medium, not a coincidence.

So the first question is what this network physically is, and the second is which test therefore applies. Reaching for a multimeter before answering the first is how a healthy optical ring gets condemned.

One safety note for the optical case: never look into the end of a fibre or a transmitter port.

Likely causes

Start here

Corroded or loose connector · easy
Damaged data bus wiring · moderate
Poor wiring splice in an aftermarket installation · moderate
Aftermarket head unit fitted in place of the original · moderate

Less common

Damaged wiring harness · 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.