U0021: Low Speed CAN Communication Bus (+) Open
What to do
Everything probably works, and that is expected rather than reassuring.
This part of your car's network runs on two wires and is designed to keep going on one if the other breaks. One has broken. The car switched to the survivor and carried on, which is why you have noticed nothing.
The reserve has now been used up. The next fault is the one that stops things — and by then this break will be months older and harder to find. Fixing one broken wire on a working car is a small job; finding two on a car that has stopped is not.
One free check worth doing: open and close each door and the tailgate slowly, and see whether anything changes. The wiring passes through a rubber concertina that is flexed every time, and it breaks inside insulation that still looks perfect.
Otherwise mention damp carpets, water in the boot, or any work involving trim, seats or a stereo installation.
What this code means
The positive line of the low speed data bus is open circuit. See U0003 for what an open is and U0019 for the fact that governs this whole family: this network is designed to carry on using the other wire.
So the expected presentation is a vehicle that works perfectly with a code stored, and the correct reading of that is not "nothing is wrong" but "the network is now running on one wire."
Which changes what the repair is worth. Fixing a single broken conductor while the vehicle still works is a small job. Fixing it after the second wire fails means finding two faults, one of them months old, on a vehicle that has stopped — and the older damage is by then corroded and harder to trace.
Where the break is follows U0012's list, because this network shares the body's routes: the door harness boot first, then the tailgate hinge, then the connectors where the loom crosses into each separate assembly.
And the free check is the same: does anything change as a door or the tailgate is moved.
Likely causes
Start here
- Broken wire in the door harness boot · moderate
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Damaged wiring harness · moderate
Less common
- Water ingress into connector · moderate
- Poor wiring splice in an aftermarket installation · 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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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.
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