Network, generic

U0034 Vehicle Communication Bus A (-) Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U0034 flags the (-) wire of Vehicle Communication Bus A pulled toward ground. On a high-speed-type CAN, the layout Audi uses for the sensor/hybrid CAN behind this slot, that wire is CAN-Low, which normally drops by about a volt for dominant bits. onsemi found that with CAN-Low grounded, frames still pass, but the bus levels are distorted and the sending node draws more current, because its high-side driver now feeds the termination against ground. NXP lists this case among the hidden failures that go unnoticed without transceiver diagnosis. Expect intermittent errors or one module reporting the code rather than a dead network. No manufacturer documents U0034 publicly. A resistance near zero between CAN-Low and ground with the battery off confirms it.

Likely causes

(-) wire insulation damaged and touching ground
Harness pinched by a bracket or panel
Faulty transceiver pulling the (-) line to ground

Symptoms

  • Intermittent communication errors, vehicle mostly normal
  • Code reported by a single module

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–700
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

U0007U0028U0031U0033U0035

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