U0040Vehicle Communication Bus B (+) Low
- Generic OBD-II code · Network
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- Likely(+) wire chafed through against body ground
- PossibleHarness crushed by a seat rail, bracket or panel
- RareTransceiver of one module pulling the (+) line to ground
- Repair
- Hard · Usually a job for a workshop.Typical cost: €60 – €800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.