U0021
Low Speed CAN Communication Bus (+) Open
Communication network (CAN bus)
Generic OBD-II
Medium
What it means
U0021 means a module has detected an open circuit on the CAN-High (+) wire of the low-speed communication bus. This bus links body and comfort modules, so an open wire cuts off data to whichever modules sit past the break — typically causing dead or erratic convenience features and warning lights rather than an engine problem. It's a wiring-side fault: something on the CAN-High line has gone open.
Medium
Can I keep driving? Usually drivable — the low-speed bus carries body/comfort data, but an open CAN wire can silence several convenience features and trip warning lights.
Common symptoms
What you'll notice
- Warning lights on the dash
- Convenience/body features dead or erratic (locks, lighting, climate, infotainment)
- Several modules on the low-speed bus report lost communication
- Fault appears suddenly after wiring work or a collision
Common causes
Most likely first
- Open (broken) CAN-High wire on the low-speed busLikely
- Disconnected, backed-out, or corroded connector terminalCommon
- Damaged wiring from rodents, chafing, or a prior repairCommon
- Internal open in a module's bus connectionLess often
How to diagnose & fix
Work through it in order
- Scan all modules; the group reporting lost communication points to the open segment.
- Inspect the low-speed CAN-High wiring and connectors along that segment for breaks or backed-out pins.
- Measure continuity on the CAN-High line to locate the open.
- Repair the broken wire or reseat/replace the faulty connector terminal.
- Clear the code and confirm the affected modules come back online.
FAQ
Quick answers
Is it safe to drive with U0021?+
Generally yes to get it repaired — the low-speed bus doesn't run the engine. But an open bus wire can disable convenience and some safety-adjacent features, so don't leave it long.
Can I just clear U0021?+
No lasting benefit — an open circuit is a physical fault. The code returns until the broken wire or connector is repaired.
Related codes
Often seen with U0021
Read & clear U0021 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX