U0023–U0026 Low Speed CAN Communication Bus (+) shorted High (to B+)
A wire in this circuit is touching a live positive feed, so the part sees power when it shouldn't.
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 |
|---|---|---|
U0023 | Low Speed CAN Communication Bus (+) shorted High (to B+) | Investigate soon |
U0026 | Low Speed CAN Communication Bus (-) shorted High (to B+) | Investigate soon |
The definition
U0023: Low Speed CAN Communication Bus (+) shorted High (to B+).
U0023 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:
- With the connector unplugged and ignition on, check whether the signal pin already has battery voltage on it. It should not.
- Check for a chafed wire lying against a live feed, especially in looms that run alongside the starter or alternator cabling.
What actually causes this on a VAG
The official definition is "Low Speed CAN Communication Bus (+) shorted High (to B+)" - a convenience-network wire is being held at battery voltage instead of swinging up and down with data. Because that same wire runs to several modules, the effect is usually a group of comfort functions dying together - central locking, windows, mirrors, alarm - rather than one item failing on its own. A bus held live also tends to stop the car going properly to sleep, so a battery that goes flat over a weekend often turns up alongside this. One thing to be clear about, because it trips people up. The code text uses the generic speed labels - high, medium, low - while VAG's wiring diagrams name their networks by the job they do: drivetrain, convenience, infotainment. Both vocabularies appear in VAG's own diagnostics, and the speed labels do not map neatly onto the named buses. So use the code for what it reliably tells you - a data wire is being held at battery voltage - and use the list of modules that have gone missing in a full scan to tell you which bus that wire belongs to. The medium-speed versions of the same fault (U0012 open, U0013 shorted to earth, U0014 shorted to battery voltage) describe the same physical damage on a neighbouring network and are chased in exactly the same way.
Most likely causes, in order
- Accessory wiring spliced into the wrong wire. Dashcams, parking sensors, alarms, immobiliser bypasses, tow bar electrics and stereo installs are the leading cause of a data wire meeting a permanent live. Look for added wiring, scotchlocks or tape joins behind the dash, at the fusebox and around the boot, and undo the most recent addition first.
- Chafe onto a live feed inside a loom. Where a loom is squashed or vibrates against a bracket, two wires inside it can wear against each other. Flex the loom by hand at bulkhead grommets, door boots and the tailgate hinge area and look for hardened or shiny insulation.
- Water in a connector. Damp bridges pins inside a plug and can put voltage where it doesn't belong. Blocked scuttle and sunroof drains are the usual route in on VAG cars, so check for wet carpet under the front seats and around any low-mounted control units.
- A control unit that has failed internally. Diagnosed by disconnecting modules on that branch one at a time and watching the bus recover. Needs a scan tool and a wiring diagram, and should only be considered once wiring and accessories are ruled out.
Worth knowing: If the battery is also going flat, don't treat that as a separate problem and buy a new battery. A data line stuck at battery voltage keeps modules awake, and both complaints usually disappear the moment the short is found. And don't buy a control unit off the back of this code - it names a wire, not a part. Have a full scan done, note exactly which modules have gone missing, then look for the one length of loom they all have in common. That's where the damage almost always is.
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 U0023 mean?
U0023 means Low Speed CAN Communication Bus (+) shorted High (to B+). A wire in this circuit is touching a live positive feed, so the part sees power when it shouldn't.
Can I drive with U0023?
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.