Network, generic

U0021 Low Speed CAN Communication Bus (+) Open

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

CAN-High wire broken at a flex point such as a door transition
Pushed-back or spread terminal in a connector
Corrosion in a connector or splice

Symptoms

  • 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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–500
Time0.5–4 h

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

Parts involved

low speed can wiringcan high wiremodule connector

Related codes

U0003U0019U0022U0023U0024

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