U0011 Moderate Generic / SAE

U0011: Medium Speed CAN Communication Bus Performance

Network and Communication Bus Signal and Bus Off

What to do

Everything probably works, and this is still worth attending to. That is the whole point of this page.

Your car's internal network is running, but messages are arriving corrupted and having to be sent again. The system is good enough at recovering that you notice nothing at all — which is exactly why this gets ignored.

Read it as an early warning. A connector corroding but still conducting, a wire down to its last few strands, insulation nearly rubbed through — all of those cause errors for a long time before they cause a failure. This is that stage.

And there is a practical reason to act now. The fault is present today, which is the easiest possible condition to measure. Once it becomes intermittent, it is one of the most expensive things on a car to chase — it produces a flood of unrelated fault codes and then clears before anyone can look.

One question is worth answering: has anything been added to the car? A tracker, dashcam, insurance box, alarm, stereo or tow bar. Anything spliced into the network changes its electrical behaviour, and a long tail of cable to an added device is a well-known cause of exactly this — invisible at rest, and visible only as errors.

Mention any damp carpets or wet boot floor while you are at it.

What this code means

The medium speed data bus is working, and not working properly. See U0010 for what this network is and U0003 for the electrical faults.

This is the most useful code in the whole range and the one most likely to be dismissed, because everything still works.

A performance code is not a failure report. It is an error rate. Messages are arriving corrupted and being retransmitted, frames are being rejected, and the bus is spending part of its time correcting itself instead of carrying traffic. CAN is good enough at recovering that a driver notices nothing — which is exactly why this deserves attention.

So the right way to read it is as an early warning. The faults at U0012 to U0018 are the same problems after they finished developing. A connector corroding but still conducting, a conductor with three strands left, insulation nearly through, a terminator drifting out of value — each produces errors long before it produces silence.

And the practical argument for acting now rather than later: the fault is currently present, which is the hardest condition to arrange on purpose. A workshop can measure a degraded bus today. Once it fails intermittently, they cannot — and an intermittent bus fault is one of the most expensive things to chase on a vehicle, because it produces a flood of unrelated codes and then clears.

Two causes belong specifically to a degraded rather than broken bus.

Something added to the network. A device spliced onto the pair changes its electrical characteristics — the bus is a terminated transmission line rather than a wire — and a long stub of cable to an aftermarket module reflects signals back down it. That is invisible at rest and shows up only as errors.

And a marginal connection somewhere. The bus tolerates one poor joint and reports it as errors; it stops at the second.

Likely causes

Start here

Poor wiring splice in an aftermarket installation · moderate
Corroded or loose connector · easy
Damaged data bus wiring · moderate
Water ingress into connector · moderate

Less common

Failed bus terminating resistor · specialist
Intermittent wiring fault · 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.