Network, generic

U0035 Vehicle Communication Bus A (-) High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U0035 means the (-) wire of Vehicle Communication Bus A is held too high, typically shorted to battery voltage. On a two-wire high-speed-type CAN, such as Audi's sensor/hybrid CAN filed under Bus A, this is CAN-Low. onsemi's tests place CAN-Low at 12 V among the shorts that make transmission impossible: frames can no longer be sent and the controllers repeat them without success. All modules on Bus A drop out together, and units on other networks register their absence. This is the opposite of U0034, where traffic survives. No manufacturer documents U0035 publicly. Measuring CAN-Low against ground with ignition on shows battery voltage instead of roughly 1.5 to 2.5 V; chafing against a supply wire or a water-filled connector are the usual reasons.

Likely causes

(-) wire chafed against a battery or ignition feed
Water in a connector linking the (-) wire to a supply pin
Damaged transceiver driving the (-) line high

Symptoms

  • All functions on Bus A fail together
  • Lost-communication codes in modules on other networks
  • 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

U0008U0028U0032U0033U0034

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