Network, generic

U0029 Vehicle Communication Bus A Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U0029 reports that Vehicle Communication Bus A works, but not reliably; which network that is depends on the maker. In Audi's 48 V mild hybrids (A6 up to Q8, 2019-23) the engine control unit logs U002900 for its sensor/hybrid CAN, labelled there as no communication, together with U002800; the belt starter-generator is then set to freewheel, the battery is no longer charged and a yellow or red battery warning appears. Audi blames connectors not plugged in or locked properly, spread pins or shorted wires, and states that a new starter-generator does not help. The Q5 PHEV stores it after the 12 V wiring feeding its power electronics rubs on the underbody trim. GM names U0029 for 2014-20 full-size trucks and SUVs with a dash harness pinched at the left instrument panel bracket, and for 2017 trucks with a hung gateway module.

Likely causes

Connector not fully seated or locked, spread or bent pins
Harness chafed on underbody trim or pinched at a dash bracket
Faulty sensor on the bus disturbing communication (Audi brake servo pressure sensor)
Gateway module stops communicating until reset

Symptoms

  • Yellow or red battery warning, battery not charging (Audi mild hybrid)
  • Intermittent warning messages from several systems
  • Comfort or infotainment functions drop out briefly

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–900
Time0.5–5 h

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

Parts involved

bus a wiringcontrol modulemodule connector

Related codes

U0002U0020U0028U0036U0038

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.