CO-16
Often appealableClaim lacks information or has a submission error
Group CO — Contractual Obligation — an adjustment tied to the contract between the insurer and the provider. You usually cannot be billed for a CO amount.
CO-16 means the claim couldn't be processed because it's missing required information or contains a submission/billing error. Like CO-96, it almost always comes with a remark code that specifies exactly what's missing or wrong.
This is usually a correctable paperwork problem, not a coverage decision. Most CO-16 denials are resolved by the provider fixing and resubmitting the claim rather than by a formal appeal.
Got a CO-16 denial? Our free generator can draft a ready-to-send appeal letter for this denial in a few minutes. Start your appeal →
Why you're seeing CO-16
- A required field — diagnosis code, NPI, member ID, or date — was missing or invalid.
- Documentation the payer needed wasn't attached.
- A referral or authorization number was required but not included.
- The claim had a formatting or data-entry error.
Can you appeal it?
Usually fixed by a corrected resubmission from the provider rather than a formal appeal — identify the missing item from the remark code first.
What to do next
- 1Read the remark code paired with CO-16 to see exactly what's missing or wrong.
- 2Contact the provider's billing office and ask them to correct and resubmit the claim.
- 3If the missing item is something you hold, provide it to the provider or payer promptly.
- 4Only file a formal appeal if the payer refuses to accept a corrected claim.
Evidence that helps
- The EOB and its remark code specifying the missing information.
- Whatever document or data element the payer said was missing.
Frequently asked questions
Do I appeal a CO-16 or resubmit?
Usually you resubmit. CO-16 is typically a correctable billing error — the provider fixes the missing information and sends a corrected claim. A formal appeal is only needed if the payer won't accept the correction.
How do I know what's missing?
Check the remark code printed next to CO-16 on the EOB. It specifies the exact field, document, or data element that was missing or invalid.
Related guide: How to appeal a “Billing or coding error” denial →
Write your appeal now
Generate a tailored appeal letter, escalation letter, and phone script — free, private, and entirely in your browser.
Other common denial codes
Last reviewed July 18, 2026. Denial-code lists (CARC/RARC) are updated three times a year; we review these explanations against the current list.
Sources & references
These explanations are written in plain language and based on the authoritative sources below. Always confirm the specifics against your own plan documents and denial letter.
- ASC X12 — Claim Adjustment Reason Codes (CARC)
The standards body that maintains the official CARC list (the CO/PR codes), updated three times a year.
- CMS / X12 — Remittance Advice Remark Codes (RARC)
The remark codes (N-/M- codes) that accompany a CARC and give the specific reason for an adjustment.
- HealthCare.gov — Appealing a health plan decision
Official overview of internal appeals and external review rights for Marketplace and most private plans.
- U.S. Department of Labor — ERISA claims & appeals
Governs claim and appeal procedures and deadlines for most employer-sponsored (ERISA) health plans.
- Your state's insurance regulator
State agencies enforce external review and consumer protections; find yours via the NAIC directory.
Important: this is not legal or medical advice.
AppealBrain is a free, self-help tool that helps you draft an appeal letter using the information you provide. We are not a law firm, medical provider, or insurance company, and using this tool does not create an attorney–client relationship. Appeal rules and deadlines vary by plan and state — always review your own denial letter and plan documents, and consider consulting a licensed professional for your specific situation. We do not guarantee any outcome.