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U04AC Invalid Data Received From Hybrid/EV Battery Interface Control Module L

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

U04AC covers interface module L, the twelfth letter. Twelve is a common pack size: Wikipedia gives twelve modules for the larger ID.4 battery and for the Ultium pack of the Cadillac Lyriq. Where one board serves each module, L would be the last board present there. Whether a maker actually wires it that way is not public; the 2024 Volkswagen list names only A, E and I for that car, and no source names U04AC. The code means messages from L arrive but fail the controller's checks, not that L has gone quiet (U01AB) or carries the wrong software (U0345). Tracing it requires the manufacturer's scan tool and, beyond the external low-voltage connections, work on an opened high-voltage battery.

Likely causes

Interface control module L faulty
Loose plug on the internal link to module L
Fault at the pack's external low-voltage connector

Symptoms

  • High-voltage battery warning
  • Power reduced, possibly no charging

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€150–3,500
Time2–10 h

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

Parts involved

hv battery interface module l

Related codes

U01ABU0345U04ABU04AD

References: en.wikipedia.org, en.wikipedia.org, static.nhtsa.gov.