Network, generic

U01AC Lost Communication With Hybrid/EV Battery Interface Control Module M

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

U01AC belongs to battery interface control module M and is the last one GM mentions: its 2018 hybrid table ends the fault bundle BICM Loss of Comm at U01AC, i.e. thirteen boards, whereas GM's model-specific tables from 2016 to 2019 only monitor A to H. For diagnosis the order of events matters more than the letter. GM first sets the summary code U179C for one or more silent boards and only afterwards names the first board it cannot reach, so a workshop should record all stored module codes, U179C and the invalid-data and software codes of the same letter (U04AD, U0346) before clearing, then compare the count with the boards actually fitted. No manufacturer publishes a threshold or MIL type for M.

Likely causes

Chain connection between module L and module M interrupted
Battery interface control module M defective
Controller calibration expects more boards than the pack contains

Symptoms

  • Summary code for missing boards stored together with U01AC
  • Battery warning and reduced output

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€100–3,500
Time1–10 h

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

Parts involved

hv battery interface module mcommunication bus

Related codes

U01ABU01ADU04ADU0346

References: gsi.ext.gm.com, gsi.ext.gm.com, law.cornell.edu.