Network, generic

U0004 High Speed CAN Communication Bus (+) Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U0004 means the (+) wire of the high-speed CAN, CAN-High, is dragged below its valid range, usually by a short to ground. This is one of the faults the physical layer cannot absorb: a sending node is unable to produce a dominant bit, every frame attempt fails, the error counters rise and the controllers end up bus-off, as onsemi's bench measurements show. Transceivers with bus diagnosis, NXP's TJA1043 for example, recognise this short up to roughly 50 ohms. Typically all powertrain and chassis modules on the network fall silent at the same moment, accompanied by U0001 and a string of lost-communication codes. No manufacturer documents U0004 publicly; an FCA bulletin describes the same picture under U0001, caused by a harness pinched beneath a console bolt.

Likely causes

CAN-High insulation damaged and touching body ground
Harness clamped by a screw, bracket or seat rail
Defective transceiver in one module pulling CAN-High down

Symptoms

  • Network-wide communication loss, scan tool cannot connect
  • Engine, gearbox, ABS and steering warnings together
  • Vehicle may not start or run in limp mode

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–1,000
Time1–6 h

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

Parts involved

can high wirecan transceiver

Related codes

U0001U0003U0005U0007

References: onsemi.com, nxp.com, static.nhtsa.gov.