CO-26: Expenses incurred before coverage began
UPDATED JULY 2026 · U.S. PLANS · NOT LEGAL OR MEDICAL ADVICE
The payer's records say your coverage started after the date of service. Sometimes true; often an enrollment-date error.
CO-26 at a glance
- Code group: CO — Contractual Obligation — in-network providers generally cannot bill you for CO adjustments.
- Who usually fixes it: You and the billing office together.
- Worth appealing? Sometimes — depends on the facts.
What to do about a CO-26 denial
- Check your coverage effective date on your ID card, enrollment letter, or HR/marketplace account.
- If the payer's date is wrong, send proof of the correct effective date and ask for reprocessing.
- If enrollment paperwork was delayed through no fault of yours, appeal with the enrollment timeline documented.
Best next read: Free appeal letter template — the full guide (with a free letter template) for this denial type.
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