U0023 Moderate Generic / SAE

U0023: Low Speed CAN Communication Bus (+) High

Network and Communication Bus Signal and Bus Off

What to do

Everything probably still works, and this is the one fault in this group that should not wait.

Here is why. This part of your car's network can run on one wire if the other fails, so when one gets shorted to battery voltage the car simply abandons it and carries on. You notice nothing.

But the voltage is still there. It is sitting on a wire bundled with the one still in use, and connected to every computer on that network. The safety net hid the fault; it did not remove the damaging condition — and every day it stays is another day of it.

So unlike the rest of this family, get this one looked at promptly even though nothing has stopped working.

The cause is nearly always something fitted after the car was built — a tracker, dashcam, alarm, stereo, light bar or tow bar. A live feed tapped next to the network wires, or a self-stripping connector clipped onto the wrong one, causes most of these. Say what has been fitted, including by a previous owner; unplugging it and re-testing is quicker than any measurement.

And be prepared for the repair to have two stages — see U0005.

What this code means

The positive line of the low speed data bus is being held high. See U0005 for the warning that matters most — battery voltage on a bus wire can damage every transceiver on the network, so the repair may have two stages.

And here that warning has a sharper edge, because of what U0019 explains about this network.

A fault-tolerant bus responds to a wire shorted to power by abandoning it and running on the other. So the vehicle keeps working — while battery voltage sits on a conductor that is bundled with the one still in use, and connected to every module on the network.

The redundancy hides the fault. It does not remove the voltage.

Which makes this the one code in the low speed family that should not be left running on its reserve. A break can wait for a convenient appointment. A short to power is applying a damaging condition continuously, and every day it remains is another day of it.

So the instruction differs from the rest of this family: have this one seen promptly even though nothing has stopped working.

And the cause is nearly always something added — a live feed tapped beside the pair, a self-stripping connector on the wrong wire, a screw through a loom. Unplugging an accessory and re-testing is quicker than any measurement.

Likely causes

Start here

Poor wiring splice in an aftermarket installation · moderate
Wiring shorted to voltage · moderate
Faulty aftermarket alarm or immobiliser installation · moderate
Damaged data bus wiring · moderate

Less common

Damaged wiring harness · moderate
Water ingress into connector · 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

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