U0022–U0025 Low Speed CAN Communication Bus (+) shorted Low (to ground)

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

U0022: Low Speed CAN Communication Bus (+) shorted Low (to ground).

U0022 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:

1. Unplug the connector at the part and measure resistance from each signal pin to a good earth. A healthy circuit reads open, not a few ohms.
2. Follow the loom for chafe points - where it crosses a bracket, passes an edge, or flexes near a hinge or the engine mounts.
3. Look for water or corrosion in the connector. A wet plug shorts pins to each other and to the body.

What actually causes this on a VAG

The official definition is "Low Speed CAN Communication Bus (+) shorted Low (to ground)" - one wire of the convenience network is being pulled down to earth instead of carrying data. On a VAG that bus is the comfort side of the car, though the code's speed label is a rough pointer rather than a wiring diagram; which physical bus is involved is confirmed by seeing which modules have gone missing in a full scan. Grounding a data line typically knocks out a whole branch at once, so the symptoms are usually broader than a single dead item: locking, windows, mirrors and interior lights all misbehaving together. It can also keep modules awake, so a flat battery after a few days sitting is a common companion complaint.

Most likely causes, in order

1. A pinched wire from recent work. A loom trapped under a seat rail, a bolt, a trim clip or a door card is the classic cause. If the car has recently had seats out, carpets lifted, a stereo or speakers fitted, a door card off or accident repair, go back over that job first - the wire is usually crushed against something metal within inches of where someone worked.
2. Chafe against the body. Loom rubbing on a bracket, a seat frame or a sharp edge in the door aperture wears through the insulation and touches bare metal. Inspect where the loom passes through bulkheads and grommets, and flex it by hand while looking for damage.
3. Water or corrosion bridging pins. Damp inside a connector or a module conducts, which reads as a short to earth. Check for wet carpet under the front seats and in the boot, and look for green or crusty pins in low-mounted plugs.
4. A failed module dragging the line down. Confirmed by unplugging modules on that branch one at a time until the bus comes back, which needs a scan tool and a wiring diagram. Don't start here - wiring is far more common and much cheaper.

Worth knowing: Ask yourself what was last done to the car. A short to earth on a convenience network is a physical injury to a wire, and on these cars that is much more often the result of something being removed and refitted than an age-related failure. Start with the most recent job anyone did in the area, not with the parts catalogue.

Common questions

What does U0022 mean?

U0022 means Low Speed CAN Communication Bus (+) shorted Low (to ground). A wire in this circuit is touching the car's metal body, so current escapes to earth instead of reaching the part.

Can I drive with U0022?

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.

Related reading

• Several warning lights on at once: usually one cause, not many
• Do you actually need a VAG-specific scan tool?
• Key not recognised or car won't unlock: VW and Audi
• Why the same fault code has completely different causes