Network, generic

U0044 Vehicle Communication Bus B (-) High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U0044 means the (-) conductor of Vehicle Communication Bus B is held at an excessive voltage, mostly through contact with battery plus. Where Bus B is a high-speed-type CAN, the conductor is CAN-Low, and onsemi found that with CAN-Low at 12 V no frame gets out any more; the node's current even drops, to about 48 mA, because its transmit pin is driven low only some 2 % of the time while the controller retries and backs off. All Bus B members fall silent together. A fault-tolerant low-speed CAN handles the same fault differently: NXP's TJA1055 detects it after a filter delay, turns off the CAN-Low driver and listens on CAN-High only. No manufacturer documents U0044 publicly. Ignition on, CAN-Low against ground near battery voltage confirms it; look for chafing at supply wires and water in connectors.

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 B fail together (high-speed type)
  • Lost-communication codes in modules on other networks
  • Fallback modes or warning lamps 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 b wiringbus b transceiver

Related codes

U0035U0037U0041U0042U0043

References: onsemi.com, nxp.com, law.cornell.edu.