U0037 High Generic / SAE

U0037: Vehicle Communication Bus B

Network and Communication Bus Signal and Bus Off Emissions related

What to do

Expect a group of systems to have stopped or to be misbehaving, and possibly a car that will not start.

One thing on this page is worth reading even if everything gets fixed, because it explains faults that otherwise make no sense.

A car's network is not simply a pair of wires — it is a carefully shaped line, with a terminator at each end and computers connected by very short tails. A long tail causes the signal to bounce back and interfere with itself.

And here is why that matters to you: a network with that problem measures perfectly. The wiring is intact, the voltages are right, every test passes — and it still corrupts messages whenever it is busy. So you get faults nobody can reproduce, on a car that has been checked twice.

The commonest cause is something plugged into the network on a length of cable — a tracker, a dashcam, an insurance box, a fleet device. The installer did nothing obviously wrong: they connected two wires and it worked. The problems came later and nobody connected the two.

So if anything of that kind is fitted, say so, and ask for it to be unplugged and the car re-tested. That is free, takes minutes, and settles it either way.

What this code means

A fault has been detected on vehicle communication bus B. See U0028 for why the letter tells you nothing and must be resolved from the vehicle's topology first, and U0003 for the electrical faults.

This page carries the fact that explains the most puzzling failures in this range: a bus is not a wire, it is a transmission line, and its shape matters as much as its continuity.

A CAN network is a linear backbone with a terminator at each physical end, and modules connected to it by short stubs. That geometry is not stylistic. A signal travelling along the backbone is absorbed at the terminated ends; a signal that reaches an unterminated end reflects and travels back, arriving late and superimposing itself on whatever is being transmitted.

Short stubs are tolerable because a reflection from a few centimetres returns before it can do harm. A long stub is not.

Which produces the failure worth recognising, because it defeats every ordinary test. A network with a long stub measures perfectly at rest — the resistance is right, the voltages are right, the wiring is intact — and corrupts messages under traffic. It presents as a performance code, or as intermittent faults that nobody can reproduce, on a vehicle where the wiring has been checked twice.

And the commonest way a vehicle acquires a long stub is an accessory installation. A tracker, dashcam, insurance box or fleet device connected to the network on half a metre or a metre of cable is a textbook long stub — and the installer did nothing wrong electrically. They tapped two wires and the network worked. Then the vehicle developed faults nobody could connect to it.

So the question that resolves these is not what is broken but what is attached, and the test is to remove the device and re-measure the error rate — which is free, immediate, and settles it.

Likely causes

Start here

Poor wiring splice in an aftermarket installation · moderate
Damaged data bus wiring · moderate
Failed bus terminating resistor · specialist
Corroded or loose connector · easy

Less common

Another module holding the bus down · specialist
Faulty gateway module · 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

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.