U0004High Speed CAN Communication Bus (+) Low
- Generic OBD-II code · Network
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- Network-wide communication loss, scan tool cannot connect
- Engine, gearbox, ABS and steering warnings together
- Vehicle may not start or run in limp mode
- Likely causes
- LikelyCAN-High insulation damaged and touching body ground
- PossibleHarness clamped by a screw, bracket or seat rail
- RareDefective transceiver in one module pulling CAN-High down
- Repair
- Hard · Usually a job for a workshop.Typical cost: €80 – €1,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.