U0021

Low Speed CAN Communication Bus (+) Open

Q: 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.

Q: 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.

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.

Real-world scan data

Seen in

scans in the past year

Found in

<0.1%

of scans

<0.1%

of scans that found faults

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear U0021 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX