What denial codes mean a coding error?

The main ones are CO-4 for a modifier that conflicts with the procedure, CO-5 for a place-of-service mismatch, CO-6 for an age mismatch, CO-11 for a diagnosis that does not support the procedure, and CO-146 or CO-181 for a diagnosis or procedure code that was not valid on the date of service.

Should I appeal or resubmit a coding denial?

Resubmit. When the claim itself was wrong, the fix is a corrected claim sent with the replacement frequency code, not an appeal. Appeal only when the original coding was right and the payer's edit misapplied it, and attach the documentation that shows why.

Coding error denial codes

Coding denials are the most recoverable group on a remittance, because the service itself is rarely in dispute - the claim describes it in a way the payer's edits reject. CO-4 flags a modifier that conflicts with the procedure code, CO-5 a procedure that does not fit the place of service, and CO-6 a procedure that does not fit the patient's age. CO-11 means the diagnosis does not support the procedure. CO-146 and CO-181 mean the diagnosis or the procedure code was not valid on the date of service, usually after an annual code-set update.

Also described as: modifier inconsistent with the procedure; wrong place of service; procedure inconsistent with age; diagnosis does not support the procedure; invalid or deleted code.

The 6 codes and what separates them

CO-4 - The modifier does not match the procedure code billed
A modifier on the line conflicts with the procedure code, is not allowed on it, or was required and missing.
Who pays: Provider
CO-5 - The procedure does not match the place of service billed
The procedure is not billable in the setting reported by the place-of-service code.
Who pays: Provider
CO-6 - The procedure does not match the patient's age
The procedure is not appropriate or not covered for the patient's age as recorded on the claim.
Who pays: Provider
CO-11 - The diagnosis does not support the procedure billed
The diagnosis codes do not give the payer a reason it accepts for the procedure.
Who pays: Provider
CO-146 - The diagnosis was invalid for the date of service
The diagnosis code was not in effect on the date of service - often a code deleted or expanded in the October ICD-10-CM update.
Who pays: Provider
CO-181 - The procedure code was not valid on the date of service
The procedure code was not in effect on the date of service - typically one deleted or replaced in the annual CPT or HCPCS update.
Who pays: Provider

How to tell which one you have

Each code points at a specific field, so the fix starts there. CO-4: compare the modifiers on the line with the procedure descriptor and the payer's modifier policy. CO-5: check the place-of-service code against where the service actually happened, including the telehealth place-of-service codes. CO-6: confirm the date of birth on the claim matches registration, since a typo there produces age edits. CO-11: look for a documented diagnosis that never made it onto the claim. CO-146 and CO-181: check the code against the code set in effect on the date of service, not the date of billing. Each of these is normally resolved with a corrected claim rather than an appeal.

Frequently asked questions

What denial codes mean a coding error?: The main ones are CO-4 for a modifier that conflicts with the procedure, CO-5 for a place-of-service mismatch, CO-6 for an age mismatch, CO-11 for a diagnosis that does not support the procedure, and CO-146 or CO-181 for a diagnosis or procedure code that was not valid on the date of service.
Should I appeal or resubmit a coding denial?: Resubmit. When the claim itself was wrong, the fix is a corrected claim sent with the replacement frequency code, not an appeal. Appeal only when the original coding was right and the payer's edit misapplied it, and attach the documentation that shows why.

Other denial reasons

Out of network No prior authorization Not medically necessary Service not covered Bundled or not paid separately Patient not eligible or coverage ended Another payer is responsible Missing information, duplicate, or late filing 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.