U0021Low Speed CAN Communication Bus (+) Open
- Generic OBD-II code · Network
- How the car detects it
- U0021 marks a break in the (+) wire of a two-wire low-speed CAN, called CAN-High in the fault-tolerant layout of ISO 11898-3. There the wire rests close to 0 V and is pulled toward 5 V for a dominant bit, the reverse of the high-speed pattern. NXP's TJA1055 treats an interrupted CAN-High as failure 1: drivers, termination and differential reception stay unchanged, and its error pin only reacts when the gap lies between sender and receiver, so it toggles rather than staying low. Communication therefore usually continues and the code mainly records lost redundancy. GM's low-speed GMLAN uses one wire only, so a (+)/(-) split does not apply there. No manufacturer documents U0021 publicly; CARB merely demands that missing communication of digital inputs be detected and coded separately.
- What you may notice
- Usually no noticeable effect, the bus keeps running
- Occasional comfort function glitches, especially with interference
- Code stored only in modules on one side of the break
- Likely causes
- LikelyCAN-High wire broken at a flex point such as a door transition
- PossiblePushed-back or spread terminal in a connector
- PossibleCorrosion in a connector or splice
- Repair
- Medium · Usually a job for a workshop.Typical cost: €50 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.