Network, generic

U0040 Vehicle Communication Bus B (+) Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U0040 indicates the (+) line of Vehicle Communication Bus B pulled down to ground potential. On a high-speed-type CAN that is CAN-High, and onsemi's bench tests list it among the shorts that stop all transmission: each failed start of frame raises the transmit error counter by eight, the controller turns error passive after a handful of attempts and goes bus-off once the count passes 255. Meanwhile the CANH driver runs into its current limit. The same short is one of the failures that NXP's fault-tolerant TJA1055 rides out without corrupting frames. So the effect ranges from a completely silent Bus B to a degraded but working one, depending on the bus type. No manufacturer documents U0040 publicly. Measure CAN-High against ground with the battery off; chafed insulation is the usual culprit.

Likely causes

(+) wire chafed through against body ground
Harness crushed by a seat rail, bracket or panel
Transceiver of one module pulling the (+) line to ground

Symptoms

  • All functions on Bus B fail at once (high-speed type)
  • Many 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

U0031U0037U0039U0041U0043U0049

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