U0018: Medium Speed CAN Communication Bus (-) Shorted to Bus (+)
What to do
The car will probably still drive, with a group of electrical fittings dead and possibly a garage saying their diagnostic tool struggles to connect.
The two wires carrying this part of your car's internal network are touching each other. Because they carry the same message in opposite directions, joining them cancels it completely — so this part of the network stops rather than working badly.
There is one test that confirms it with no reference figures needed, which is worth knowing because most bus measurements 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. A loom crushed by a seat bolt, a boot floor fixing or a trim clip. A screw put through the harness while mounting an amplifier or a bracket. Water bridging a connector.
Water is the likeliest of those on this particular network, so if this appeared after heavy rain or you have found a damp carpet, say so — it points at a connector rather than a length of wire, and that is a much shorter search.
Check your lights and indicators before driving at night.
What this code means
The two lines of the medium speed data bus are shorted together. See U0009 for why joining the pair cancels the signal completely and for the short list of physical causes.
One measurement note belongs here, because it is the exception to U0010's warning about resistance figures.
U0010 cautions that the familiar 60 ohm reading is a high speed CAN figure and does not transfer — medium and low speed networks are terminated differently, so a value that would condemn one bus can be normal on another.
A dead short is the reading that does transfer. Near zero ohms across the pair is wrong on every bus there is, whatever its normal value. So this is the one fault in the range that can be confirmed with a meter and no reference figure at all.
Where the pair gets joined on this network follows U0009's list with the body-bus emphasis: a loom crushed by a seat bolt, a boot floor fixing or a trim clip; a self-tapper put through a harness while mounting an amplifier or a bracket; and water bridging a connector in a door, a sill or the boot.
That last is more likely here than on the high speed bus, and it means a bus fault appearing after heavy rain or a flooded footwell should be looked for at a connector rather than along a length of wire.
Likely causes
Start here
- Bus wires shorted together · moderate
- Water ingress into connector · moderate
- Damaged wiring harness · moderate
- Damaged data bus wiring · moderate
Less common
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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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
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