Network, generic

U0031 Vehicle Communication Bus A (+) Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

(+) wire chafed through against body ground
Harness trapped under a panel or bracket
Transceiver of one module shorting the (+) line internally

Symptoms

  • 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

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–800
Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

bus a wiringbus a transceiver

Related codes

U0004U0028U0030U0032U0034

References: onsemi.com, law.cornell.edu, static.nhtsa.gov.