U0023 code meaning
Low Speed CAN Communication Bus (+) High
What U0023 means
U0023 means CAN-High, the (+) conductor of a dual-line low-speed CAN, sits too high, typically shorted to battery voltage or a 5 V supply. Under ISO 11898-3 this pins the line at a level the receivers read as permanently dominant. NXP's TJA1055 handles it as failure 3 or 3a in two steps: once an initial delay expires it falls back to single-wire operation, and if the line is still too high it moves over to CAN-Low, switches its CAN-High driver off and weakens that line's termination; detection takes about 1.6 to 8 ms. Traffic continues on one wire, though electromagnetic robustness is worse in that mode. No manufacturer documents U0023 publicly; GM's low-speed GMLAN, being a single conductor, offers no (+)/(-) pair. CAN-High measuring near battery voltage against ground instead of its resting level close to 0 V points to the short.
U0023 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 U0023
- Comfort functions mostly still work, occasional glitches
- Increased sensitivity to interference, e.g. with radio transmitters nearby
- Code stored in several body modules
Common causes of U0023
- Most likelyCAN-High chafed against a battery feed or ignition wire
- PossibleWater in a connector bridging CAN-High to a supply pin
- Less likelyRetrofitted accessory tapped into the wrong wire
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 wire
What each part of U0023 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 23 | The exact fault in that area. |
What to do next
- Note U0023 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 U0023
What does U0023 mean?
U0023 means "Low Speed CAN Communication Bus (+) High". U0023 means CAN-High, the (+) conductor of a dual-line low-speed CAN, sits too high, typically shorted to battery voltage or a 5 V supply. It is a generic code, so it means the same on every make.
Is it safe to drive with U0023?
U0023 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U0023 fail an emissions test?
U0023 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 U0023 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will U0023 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.