Network, generic

U0039 Vehicle Communication Bus B (+) Open

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U0039 reports a break in the (+) conductor of Vehicle Communication Bus B, which on a CAN-type network is CAN-High. Deutz assigns this code to the third CAN channel of its EMR L1 engine controller and lists it for main and secondary ECU simply as an open circuit on CAN 3, without naming the wire. On a classic high-speed CAN the interruption stays invisible to the transceiver; NXP notes that even diagnosing transceivers cannot flag a wire break, so the module infers it from partners falling silent. A fault-tolerant low-speed CAN behaves differently: NXP's TJA1055 keeps sending and receiving after a CAN-High break without spoiling frames already on the line. In practice, functions beyond the break drop out while others carry on. Look for unplugged connectors, backed-out pins and breaks near moving parts.

Likely causes

(+) wire of Bus B broken, often at a flex point
Terminal backed out or spread in a connector
Connector left open after repair work

Symptoms

  • Functions of modules beyond the break unavailable
  • Lost-communication codes for individual modules

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–700
Time0.5–5 h

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

Parts involved

bus b wiringmodule connector

Related codes

U0030U0037U0040U0041U0042U0048

References: serdia.deutz.com, serdia.deutz.com, nxp.com, nxp.com.