U0027 Moderate Generic / SAE

U0027: Low Speed CAN Communication Bus (-) Shorted to Bus (+)

Network and Communication Bus Signal and Bus Off

What to do

Unlike the other faults in this group, this one actually stops things working — and that difference is itself the clue.

This part of your car's network is designed to keep running on one wire if the other fails, which is why most faults here leave you noticing nothing at all. But this one joins the two wires together, so there is no good wire left to fall back on — and the network genuinely goes down.

So if things have actually stopped working rather than quietly carrying on, this is very likely why.

There is one test that confirms it with no reference figures needed, which matters because most measurements on this network need the manufacturer's data: a dead short reads close to zero ohms across the pair, and that is wrong on every network there is.

Two wires twisted together only touch for physical reasons, and the list is short. A loom crushed by a seat bolt, a boot floor fixing or a trim clip. A screw put through the harness during a stereo, alarm or tow bar installation. Water bridging a connector in a door or a sill. Rodents.

The car will probably still drive. Check your lights, indicators and brake lights before driving at night.

What this code means

The two lines of the low speed data bus are shorted together. See U0009 for why joining a differential pair cancels the signal completely.

This is the one fault in the low speed family that the fallback cannot survive, and saying so is the point of this page.

U0019 explains that this network keeps running on one wire when the other fails — open, shorted to earth, shorted to power. Every one of those leaves a good conductor to fall back to.

Two wires shorted together leaves none. Each is now tied to the other, so neither can be driven independently, and single-wire mode has nothing to run on. So unlike everything else in this family, the network genuinely stops.

Which reverses the usual reading of a low speed code. Elsewhere in this family, everything working is expected. Here, things have stopped — and the contrast is itself diagnostic: a low speed bus that has actually gone down, rather than quietly degraded, is this fault far more often than any of the others.

The confirmation is U0009's and it needs no reference figures. Near zero ohms across the pair is wrong on every network there is, whatever its normal terminated value — which matters here, because U0019 warns that the familiar 60 ohm figure means nothing on this bus.

And the causes are physical and short: a loom crushed by a seat bolt or a trim clip, a screw put through a harness during an installation, water bridging a connector in a door or a sill, rodent damage that stripped both conductors at one point.

Likely causes

Start here

Less common

Poor wiring splice in an aftermarket installation · moderate
Blocked door drain letting water into the module · 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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