OBD-II code · Network
U0021 Low Speed CAN Communication Bus (+) Low
CAN-High on the low-speed comfort bus is shorted to ground. Bus signaling is impaired or lost; affected comfort modules drop offline.
Is it safe to drive with U0021?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyCAN-High shorted to ground
- Less likelyGateway module routing CAN messages incorrectly
- Less likelyCAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.8–4 h
- Emissions test
- Usually unaffected
- Parts involved
- Low Speed Can Wiring
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.