U0003 High Generic / SAE

U0003: High Speed CAN Communication Bus (+) Open

Network and Communication Bus Signal and Bus Off Emissions related

Do not keep driving

The car will probably not start, or will have stopped, with a dashboard either dead or lit up with warnings from everything at once.

That looks catastrophic and it is one broken wire.

The computers in your car talk to each other over a pair of wires twisted together. They are not two separate connections — they carry the same message twice, in opposite directions, and the computers read the difference between them. So the second wire is not a spare: break either one and the whole network stops. Every computer then reports that it cannot hear any of the others, which is why the scan tool produces an alarming list.

Do not let anyone start replacing computers on the strength of that list. They are almost certainly all fine.

There is a free test that settles this in two minutes, and it is worth asking for by name. With the ignition off, measure the resistance across pins 6 and 14 of the diagnostic socket. A healthy bus reads about 60 ohms. About 120 means the wiring is broken somewhere between the two ends of the network — which is exactly this fault, and it points at a wire rather than a part.

Mention anything that might have disturbed the wiring: accident repair, a stereo or alarm installation, a tow bar, or signs of rodents in the engine bay. Those are where breaks actually happen.

The car will need recovering rather than driving.

What this code means

The positive line of the high speed data bus is open circuit. This page establishes the seven electrical faults that repeat on every bus in this range, so the pages that follow can stay short. See U0001 for what the high speed bus is.

Start with why one broken wire stops everything, because it is the fact that makes this range make sense.

The bus is a pair, and it does not carry one signal on each wire. It carries the same signal twice, inverted. CAN-H rises while CAN-L falls, by the same amount, at the same instant. A receiver reads the difference between them and ignores anything that happened to both — which is why the pair is twisted together, and why a vehicle full of ignition coils, injectors and motors does not corrupt it.

So the second wire is not a spare. It is half of the measurement. An open on either one leaves no difference to read, and the entire network stops — every module reporting lost communication with every other, and usually a vehicle that will not start.

Which brings the test that is worth more than everything else on this page, costs nothing, and is decisive.

Switch the ignition off, wait for the network to sleep, and measure resistance across pins 6 and 14 of the diagnostic socket — CAN-H and CAN-L. A healthy high speed bus reads about 60 ohms. That number is not arbitrary: the backbone is terminated at each of its two ends by a 120 ohm resistor, and two 120s in parallel are 60. So:

About 60 ohms — the bus is electrically intact. The fault is elsewhere.
About 120 ohms — one terminator is missing, or the backbone is broken between the two of them. This is what an open reads, and it is the single most useful result in the range.
Open circuit, or many thousands of ohms — the pair is severed, or both terminators are gone.
Near zero — the two wires are shorted together. See U0009.

One consequence of that reading is worth knowing before a quote is accepted. The terminating resistors are not usually separate components. They are inside two modules at the physical ends of the backbone — commonly the engine module and one other. So a steady 120 ohms means either a broken backbone or a module that is no longer terminating, and the second is a module fault reached by measurement rather than by guesswork.

Where opens actually occur, in order: connectors and splices — the bus is spliced, not daisy-chained through plugs, so there are joints in the loom nobody can see; harness sections that flex, particularly through door boots and tailgate hinges; rodent damage, which is common and obvious once looked for; and accident repair, where a loom was cut and rejoined.

And the standing warning from U0100 applies at its strongest here: a scan tool listing lost communication with every module on the vehicle is describing one broken wire, not a dozen failed computers.

Likely causes

Start here

Damaged data bus wiring · moderate
Corroded or loose connector · easy
Damaged wiring harness · moderate
Failed bus terminating resistor · specialist

Less common

Poor wiring splice in an aftermarket installation · 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

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.