U0031Vehicle Communication Bus A (+) Low
- Generic OBD-II code · Network
- Can trigger limp mode
- How the car detects it
- U0031 indicates a ground short on the (+) conductor of Vehicle Communication Bus A. Where Bus A is a high-speed-style two-wire CAN, such as the sensor/hybrid CAN Audi assigns to this slot in its 48 V mild hybrids, the (+) wire is CAN-High, which has to rise by about a volt to form a dominant bit. Grounded, it cannot: onsemi's measurements list CAN-High to ground among the shorts under which transmission becomes impossible, so the controllers keep retrying in vain. Every node on Bus A then falls silent together, and modules on other networks record lost-communication codes for them. No manufacturer documents U0031 publicly. With the battery disconnected, a few ohms or less between CAN-High and ground locate the short; chafed harness sections are the usual cause.
- What you may notice
- All functions served by Bus A fail at once
- Many lost-communication codes in other modules
- Warning lamps or fallback modes depending on the modules involved
- Likely causes
- Likely(+) wire chafed through against body ground
- PossibleHarness trapped under a panel or bracket
- RareTransceiver of one module shorting the (+) line internally
- Repair
- Hard · Usually a job for a workshop.Typical cost: €60 – €800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.