U0021–U0024 Low Speed CAN Communication Bus (+) Open
The ECU has logged a fault on U0021. The definition below is the exact wording the standard uses for this code.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
U0021 | Low Speed CAN Communication Bus (+) Open | Investigate soon |
U0024 | Low Speed CAN Communication Bus (-) Open | Investigate soon |
The definition
U0021: Low Speed CAN Communication Bus (+) Open.
U0021 is a network code (communication between control modules), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.
What to check first
Cheapest and fastest checks first:
- Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
- Check the connector at the part for corrosion, and that it is fully seated.
- Clear the code and see whether it returns, and under what conditions.
What actually causes this on a VAG
The official definition is "Low Speed CAN Communication Bus (+) Open". On a VAG the low-speed bus is the convenience or comfort side of the car - doors, locking, windows, mirrors, alarm, seats and similar. One caution about the name: the code text uses the generic speed labels (high, medium, low) while VAG's wiring diagrams name their networks by job - drivetrain, convenience, infotainment - and the two vocabularies don't map neatly onto each other. Both appear in VAG's own diagnostics, so treat the speed label as a pointer rather than a wiring diagram, and let the list of modules that have gone missing tell you which physical bus is really involved. "Open" means a data wire is broken or unplugged rather than shorted, so a module has simply gone silent. You'll usually notice it as symptoms rather than as a warning light: one door not locking or its window not working from the driver's switch, an alarm that triggers for no reason, or a mirror that has stopped folding.
Most likely causes, in order
- Broken wire in a door loom. Door harnesses flex thousands of times and eventually a strand-by-strand break develops inside the insulation, usually in the concertina boot between the door and the A-pillar. Open the door, squeeze and flex the boot, and look for wires that have gone stiff, green or bare. This is the classic VAG failure in this circuit and the first place to look.
- A disturbed or corroded connector. Plugs under the seats, behind door cards and behind the dash get knocked loose during trim work, seat removal or a stereo fit. Check that plugs are fully home and locked, and look for pushed-back or corroded pins rather than assuming the wire is broken.
- A module that has lost its power or earth. A control unit with a blown fuse or a corroded earth looks exactly like a broken data wire from the outside. Check the relevant fuse and the module's supply before opening up any loom.
- Water damage to a module or its plug. Blocked drains soak footwells and boot floors. Anything mounted low in the car is worth checking for damp, and a wet module can be dead in one direction only, which reads as an open circuit.
Worth knowing: An open circuit silences whatever sits beyond the break, so the list of missing modules in a full scan is most of the diagnosis. If one door has gone quiet, the fault is in that door's loom; if a whole branch has gone, look further upstream. One thing to expect: a break part-way along a two-wire network can also cut the bus off from one of its end resistors, so the modules that are still connected may go flaky and intermittent rather than staying perfectly normal - don't let that odd behaviour send you chasing a second fault. And replacing the silent module fixes nothing when the wire feeding it is broken.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
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Common questions
What does U0021 mean?
U0021 means Low Speed CAN Communication Bus (+) Open. It is logged when the ECU sees this circuit outside its expected operating window.
Can I drive with U0021?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.