What is the denial code for timely filing?

CO-29 means the claim reached the payer after its timely filing limit. It is rarely overturned unless you can prove an earlier timely submission, such as a clearinghouse acceptance report or payer acknowledgment dated inside the window. Filing limits vary by payer and often by plan type.

What is the denial code for a duplicate claim?

CO-18 means the payer already has a claim it considers identical for the same patient, provider, date of service, and procedure. Check the original claim's status before resubmitting, and if you meant to change the claim, send it as a corrected claim rather than a new one.

What is the denial code for missing information?

CO-16 is the general code for missing or invalid information, and the remark code on the same line says which field is wrong. CO-252 is used when a required attachment was not submitted, and CO-226 when the payer requested information and did not receive it in time.

Administrative denial codes

Administrative denials are about the claim, not the care - the payer has not judged whether the service should be paid. CO-16 means required information is missing or invalid, and a remark code on the same line names the field. CO-252 means the payer needs an attachment it did not receive, and CO-226 that it asked for information and got no answer in time. CO-18 means the payer already has an identical claim. CO-29 is the costly one: the claim arrived after the payer's timely filing deadline, and it is rarely overturned without proof of an earlier submission.

Also described as: missing or invalid information; duplicate claim; timely filing limit expired; medical records requested; attachment required.

The 5 codes and what separates them

CO-16 - Claim or service lacks required information
Information the payer needs to adjudicate is missing or invalid; the remark code identifies the field.
Who pays: Provider
CO-252 - An attachment or additional documentation is required
The payer needs a supporting document, such as an operative note or a primary payer's remittance, before it will adjudicate.
Who pays: Provider
CO-226 - Requested information was not received from the provider
The payer requested information from the provider and did not receive it within its response window.
Who pays: Provider
CO-18 - Exact duplicate claim or service
The payer already has a claim it treats as identical for the same patient, provider, date, and procedure.
Who pays: Provider
CO-29 - The time limit for filing has expired
The claim reached the payer after its timely filing limit, so it was not considered on the merits.
Who pays: Provider

How to tell which one you have

CO-16, CO-252, and CO-226 are recoverable: send exactly what the remark code or the request asked for, as a corrected claim or a records submission, inside the payer's window. CO-18 is usually a resubmission that should have been a corrected claim or a status check, so confirm whether the original claim is paid, pending, or denied before sending anything else. CO-29 needs evidence the claim was filed on time, such as a clearinghouse acceptance report or a payer acknowledgment dated within the limit. A claim rejected at the front end was never received, so it does not stop the clock.

Frequently asked questions

What is the denial code for timely filing?: CO-29 means the claim reached the payer after its timely filing limit. It is rarely overturned unless you can prove an earlier timely submission, such as a clearinghouse acceptance report or payer acknowledgment dated inside the window. Filing limits vary by payer and often by plan type.
What is the denial code for a duplicate claim?: CO-18 means the payer already has a claim it considers identical for the same patient, provider, date of service, and procedure. Check the original claim's status before resubmitting, and if you meant to change the claim, send it as a corrected claim rather than a new one.
What is the denial code for missing information?: CO-16 is the general code for missing or invalid information, and the remark code on the same line says which field is wrong. CO-252 is used when a required attachment was not submitted, and CO-226 when the payer requested information and did not receive it in time.

Other denial reasons

Out of network No prior authorization Not medically necessary Service not covered Bundled or not paid separately Coding mismatch or invalid code Patient not eligible or coverage ended Another payer is responsible Provider not eligible or not credentialed Benefit or unit limit reached Deductible, coinsurance, or copay Paid less than billed

Every denial code in one table

Last reviewed October 3, 2026.

These are original plain-English summaries written for reference, not the official X12 code descriptions. Payer handling of any code varies by contract - always verify against the remittance advice and your payer agreement before adjusting a claim.