Network, generic

U0023 Low Speed CAN Communication Bus (+) High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

CAN-High chafed against a battery feed or ignition wire
Water in a connector bridging CAN-High to a supply pin
Retrofitted accessory tapped into the wrong wire

Symptoms

  • Comfort functions mostly still work, occasional glitches
  • Increased sensitivity to interference, e.g. with radio transmitters nearby
  • Code stored in several body modules

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–600
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

low speed can wiringcan high wire

Related codes

U0005U0019U0021U0022U0026

References: nxp.com, law.cornell.edu, saabwisonline.com.