U0021 code meaning
Low Speed CAN Communication Bus (+) Open
What U0021 means
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.
U0021 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0021
- 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
Common causes of U0021
- Most 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
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Low speed CAN wiringCAN high wireModule connector
What each part of U0021 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 21 | The exact fault in that area. |
What to do next
- Note U0021 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0021
What does U0021 mean?
U0021 means "Low Speed CAN Communication Bus (+) Open". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U0021?
U0021 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U0021 fail an emissions test?
U0021 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0021 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will U0021 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.