Claim adjustment reason code
Denial code CO-166 meaning
CARC 166, usually seen as CO-166: The claim was sent after this payer stopped handling claims for this plan.
Run it in Rivvi
Check the codes behind a denial
Upload a claims export. Rivvi flags ICD-10 and HCPCS codes that are not billable, replaced, or unspecified when a specific code exists.
Check my exportFree to start. Opens Rivvi, the AI workspace for healthcare teams.
- Reason code
- CARC 166
- Short name
- Sent after the payer's responsibility ended
- Plain meaning
- The claim was sent after this payer stopped handling claims for this plan.
- Group codes
- CO-166: 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
- The plan moved to a new administrator.
What to check before resubmitting
- Bill the new administrator or payer.
Related reason codes
Related pages
Questions
What does denial code CO-166 mean?
The claim was sent after this payer stopped handling claims for this plan. This is Rivvi's plain-English summary of CARC 166. X12's exact wording is on x12.org.
How do I fix CO-166?
Bill the new administrator or payer.
Can I bill the patient for CO-166?
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 166?
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.