Triage: Correct and resubmit
The CPT or HCPCS code on the claim was deleted, replaced, or simply not valid on that date of service.
The classic chiropractic example is electrical stimulation coding: some payers require G0283 where others require 97014, and billing the wrong one of the pair produces exactly this denial. Deleted codes lingering in fee schedules after an annual CPT update do it too.
A payer requires G0283 in place of 97014 for unattended electrical stimulation. The fee schedule was never updated for that payer, so every visit with e-stim generates a CO-181. One payer rule entry substituting the code fixes the denial permanently, and the corrected claims pay.
Per payer rules handle known substitutions like 97014 versus G0283 automatically at claim generation, and a CO-181 that slips through routes to correct and resubmit with the substitution suggested.
General information for billing teams, not legal or payer specific advice. CARC definitions belong to X12; payer policies vary and control. Verify against the payer contract before acting on any individual claim.