Denial code library

A claim adjustment reason code (CARC) tells you why a payer paid less than billed or nothing at all; the remark code (RARC) next to it narrows the reason down.

Each page here gives the plain-English meaning in our own words, the common causes, the fix, how to stop it recurring and the service that owns it. The official code text and the full list are maintained by X12.

Last reviewed October 21, 2026 by the Remitwright billing team. Editorial policy

Claim adjustment reason codes with the plain-English meaning, the code group and the Remitwright service that owns the fix
Code	Plain-English meaning	Group	Owned by
CO-4	The modifier does not fit the procedure code, or a required modifier is missing	Contractual obligation	Medical coding
CO-5	The place of service or bill type does not fit the procedure code on the line	Contractual obligation	Claim submission
CO-11	The procedure billed does not fit the diagnosis it points to on the claim	Contractual obligation	Medical coding
CO-15	Authorization number missing or not matched to the service; X12 retired this code in 2018	Contractual obligation	Prior authorization
CO-16	The claim is missing information the payer needs to process it	Contractual obligation	Claim submission
CO-18	The payer already has this claim or line and treats the new one as an exact duplicate	Contractual obligation	Claim submission
CO-19	The payer treats the injury as job-related and points you to the workers' comp carrier	Contractual obligation	Workers' compensation billing
CO-22	Another plan appears to be primary; the payer wants that plan's payment before it pays	Contractual obligation	Eligibility verification
CO-24	A managed care plan or capitation contract covers this patient's care, not this claim	Contractual obligation	Eligibility verification
CO-29	The claim arrived after the payer's deadline for filing an original or corrected claim	Contractual obligation	A/R follow-up
CO-45	The difference between your charge and the payer's allowed amount under its fee schedule	Contractual obligation	Payment posting
CO-49	The plan excludes routine checkups and the screening work done as part of them	Contractual obligation	Patient billing support
CO-50	The payer decided the service was not medically necessary for the diagnosis billed	Contractual obligation	Denial management
CO-96	The service or item is not a covered benefit under the member's plan or the payer's rules	Contractual obligation	Eligibility verification
CO-97	The payer treats this line as already paid for through another service it processed	Contractual obligation	Medical coding
CO-109	This payer is not responsible for the claim; it belongs with another payer or contractor	Contractual obligation	Eligibility verification
CO-119	The member has used up the plan's limit on visits, units or dollars for this service	Contractual obligation	Eligibility verification
CO-146	An ICD-10-CM code on the claim was not billable under the code set in force that day	Contractual obligation	Medical coding
CO-150	Your records, as the payer read them, fall short of the level of service you billed	Contractual obligation	Claim appeals
CO-151	The units or frequency billed exceed what the documentation or the payer's edits support	Contractual obligation	Charge entry
CO-167	The payer does not cover services for this diagnosis under the plan or its policy	Contractual obligation	Medical coding
CO-170	The payer does not pay this provider type or specialty for this service	Contractual obligation	Credentialing
CO-181	The CPT or HCPCS code billed was not in effect, or no longer existed, on the service date	Contractual obligation	Charge entry
CO-183	The payer will not accept the clinician named as ordering or referring for this service	Contractual obligation	Denial management
CO-185	The payer will not pay the clinician named as rendering for this service	Contractual obligation	Credentialing
CO-197	No authorization, precertification or notification was on file for the service	Contractual obligation	Prior authorization
CO-198	The service went beyond the units, visits or dates the authorization allowed	Contractual obligation	Prior authorization
CO-204	The patient's benefit plan does not cover this service, item or drug	Contractual obligation	Eligibility verification
CO-226	The payer asked you for records or information and did not get a usable response	Contractual obligation	Denial management
CO-236	Two codes billed the same day conflict under NCCI or payer bundling edits	Contractual obligation	Medical coding
CO-242	The plan pays only in-network clinicians for this member, and you are outside that network	Contractual obligation	Out-of-network billing
CO-252	The payer needs an attachment or documentation before it will decide the claim	Contractual obligation	Claim submission
CO-253	A federal budget sequestration cut reduced the payer's payment on this claim	Contractual obligation	Payment posting
CO-288	The plan wanted a referral from the member's primary care provider and found none	Contractual obligation	Prior authorization
CO-A1	A general denial whose actual reason is in the remark code sent with it	Contractual obligation	Denial management
CO-B7	The provider lacked the enrollment or certification this service needed on that date	Contractual obligation	Payer enrollment
CO-B9	The patient elected hospice, so Medicare expects the hospice to cover related care	Contractual obligation	Medicare billing
CO-B15	This code pays only alongside a qualifying primary service, and none has been paid	Contractual obligation	Medical coding
CO-B16	The patient does not meet the payer's test for a new patient visit	Contractual obligation	Medical coding
OA-23	Informational only, the part of the charge an earlier payer already paid or adjusted	Other adjustment	Payment posting
PR-1	Some or all of the allowed amount went to the deductible, and the patient owes that part	Patient responsibility	Patient billing support
PR-2	The patient owes a share of the allowed amount as coinsurance	Patient responsibility	Patient billing support
PR-3	The patient's fixed copayment for the visit or service is due from the patient	Patient responsibility	Patient billing support
PR-27	The patient's coverage with this payer had ended before the date of service	Patient responsibility	Eligibility verification
PR-31	The payer cannot match the patient on the claim to any member it covers	Patient responsibility	Eligibility verification

Meanings are paraphrased. The official descriptions are on the X12 claim adjustment reason code list and the remittance advice remark code list.

Denial code library

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