U0009 High Generic / SAE

U0009: High Speed CAN Communication Bus (-) Shorted to Bus (+)

Network and Communication Bus Signal and Bus Off Emissions related

Do not keep driving

The car may crank but not run, with a dead dashboard, and a garage that says their diagnostic tool cannot connect at all.

That combination is this fault almost exactly. The two wires carrying your car's internal network are touching each other, and because they carry the same message in opposite directions, joining them cancels the signal completely. The network does not work badly — it stops. Your lights, horn and starter still work because they are not on it.

There is a free test that confirms it beyond doubt, and it is worth asking for by name: the resistance across pins 6 and 14 of the diagnostic socket. A healthy car reads about 60 ohms; a car with this fault reads close to zero. No interpretation needed.

Two wires that are twisted together only touch for physical reasons, and the list is short. A loom crushed by a panel, a seat bolt or a bracket. A screw or trim clip put through the harness during a repair or an accessory installation. Water bridging a connector. Rodents.

And one free check first if you tow: unplug the trailer and look inside the socket. A trailer socket full of water is a recognised cause on vehicles whose network runs nearby, and that takes a minute rather than an afternoon.

The car will need recovering. Do not let computers be replaced — they are almost certainly all fine.

What this code means

The two lines of the high speed data bus are shorted together. See U0003 for the pair and the resistance test.

This is the fault the resistance test identifies most decisively, and it is worth knowing because the reading is unmistakable. A healthy bus reads about 60 ohms across pins 6 and 14 of the diagnostic socket. A pair shorted together reads close to zero. There is no ambiguity and no interpretation required.

And the consequence is total. The two wires carry the same signal inverted, so joining them means each cancels the other exactly — there is no difference left to read, anywhere on the vehicle. The network does not degrade; it stops.

Which is why the presenting picture is a vehicle that appears completely dead electronically while its lights, horn and starter may all still work. Those are not on the bus. A car that cranks but will not run, with a dead dashboard and a scan tool that cannot connect, is describing this exactly.

Where two wires that are twisted together end up joined is nearly always physical, and the list is short:

A crush. A loom trapped by a panel, a seat bolt, a bracket or a jacking point.
A fastener through the harness. A screw fitted during a repair, a trim clip, a self-tapper from an accessory installation.
Water bridging a connector. Rarely enough on its own to read as a dead short, but enough to stop the bus — and it is why a bus fault appearing after heavy rain or a flooded footwell should be looked for at a connector rather than along a wire.
Rodent damage, where the insulation is gone from both conductors at the same point.

And one route worth naming because it is common on towing vehicles. A trailer socket full of water, on a vehicle whose bus is routed nearby, is a recognised cause — so unplug any trailer and check the socket before searching the vehicle. That is free and it settles a proportion of these.

Likely causes

Start here

Less common

Poor wiring splice in an aftermarket installation · moderate
Corroded or loose connector · easy

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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The car — make, model, year and engine. The same fault behaves differently on each.
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What actually fixed it — or that nothing did, which is worth knowing too.