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EOB denial-code decoder

What does your EOB denial code mean?

Paste any CARC or RARC code from your Explanation of Benefits to see the plain-English meaning, the typical cause, and concrete next steps — verified against X12 publications, with a source link on every code.

Browse all CARC + RARC codes26 top codes written · verified June 16, 2026

Common denial codes

Start with the codes people ask about most

These are among the most frequently seen CARC and RARC codes. Tap any one for the full breakdown.

CO-109CARCCOThis insurer is not the right payer for this claim — it should go to a different plan or contractor.CO-11CARCCOThe insurer says the diagnosis on the claim does not justify the procedure that was billed.CO-119CARCCOYou have reached a benefit limit for this service in the current period (for example, a yearly visit cap), so no more is covered for now.CO-16CARCCOThe claim is missing required information or contains a billing error, so the insurer could not finish processing it. A Remittance Advice Remark Code (an N- or M- code) usually appears alongside this one to say exactly what is missing.CO-18CARCCOThe insurer believes this exact claim or service was already submitted, so it was not paid a second time.CO-197CARCCOA required prior authorization or pre-certification was not on file before the service, so the claim was denied.CO-204CARCCOThis specific service, equipment, or drug is not a benefit under your current plan.CO-22CARCCOThe insurer believes another insurance plan should pay before it does, under coordination-of-benefits rules.CO-252CARCCOThe insurer needs additional documentation (such as medical records) before it can decide the claim. A remark code usually says exactly what.CO-29CARCCOThe claim was submitted after the insurer's filing deadline, so it was not paid.CO-31CARCCOThe insurer could not match you to an active policy with the information on the claim.CO-4CARCCOThe insurer says the billing modifier attached to the procedure does not match the procedure code itself.

How it works

Three steps from a cryptic code to a clear next move

  • Paste your denial code

    Type or paste the CARC/RARC code from your EOB — formats like CO-22, co22, or “CO 22” all work.

  • Read the plain-English meaning

    See what the code means, the official X12 description, and what typically causes it.

  • Follow concrete next steps

    Get an ordered checklist of what to do — confirm details, contact the right office, or correct the claim.

  • Scaffold an appeal if it applies

    If the denial is typically appealable, build an appeal letter matched to your plan type and state.

A denied claim is rarely the last word.

Most denials are administrative — a missing modifier, an eligibility mismatch, a prior authorization not on file. Understanding the code is the first step toward resolving it.

Decode a code

Appeal-letter scaffolding

Need to write an appeal letter?

If your denial is appealable, the generator scaffolds a letter matched to your plan type — ERISA, fully-insured, Medicare Advantage, individual ACA, or Medicaid — with the right regulation cited and a state-DOI overlay where it applies. Everything stays in your browser.

Start an appeal letter
  • Plan-awareMatches the appeal framework to your plan type.
  • Pre-filledPulls the code and X12 description into the letter.
  • PrivateNothing is POSTed — print or save as PDF locally.
  • CitedFooter pins the governing regulation.

Questions

Frequently asked