Claim adjustment reason code
Denial code CO-4 meaning
CARC 4, usually seen as CO-4: The procedure code needs a modifier that is not on the claim line, or the modifier billed does not go with that code.
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- Reason code
- CARC 4
- Short name
- Modifier missing or does not fit the code
- Plain meaning
- The procedure code needs a modifier that is not on the claim line, or the modifier billed does not go with that code.
- Group codes
- CO-4: Contractual obligation. The provider absorbs the amount under its contract or the payer's rules. Do not bill the patient for it.
Plain-English summary written by Rivvi. X12 owns the CARC list and its exact wording, which is not reproduced here. Read it on the X12 CARC list (list dated November 1, 2025, checked October 23, 2026). This page does not decide how a claim is paid.
Common causes
- A required modifier such as 26, TC, RT, LT or a telehealth modifier is missing.
- The modifier billed is not valid for the procedure code.
What to check before resubmitting
- Check the payer's modifier rules for the code, and the CMS HCPCS file for Level II modifiers.
- Correct the modifier and send a corrected claim.
Remark codes that often come with it
Related reason codes
Related pages
Questions
What does denial code CO-4 mean?
The procedure code needs a modifier that is not on the claim line, or the modifier billed does not go with that code. This is Rivvi's plain-English summary of CARC 4. X12's exact wording is on x12.org.
How do I fix CO-4?
Check the payer's modifier rules for the code, and the CMS HCPCS file for Level II modifiers. Correct the modifier and send a corrected claim.
Can I bill the patient for CO-4?
With group code CO, no. CO means the provider absorbs the amount under its contract or the payer's rules.
Which group codes come with CARC 4?
Most often CO (contractual obligation). The group code, not the reason code, says who owes the amount.
What is the difference between a CARC and a RARC?
A claim adjustment reason code (CARC) says why a claim line was paid differently than billed. A remittance advice remark code (RARC) adds detail, such as which field is missing.