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CO-166 Denial Code: Payer's Responsibility Has Ended

CO-166 means the services were submitted after this payer's responsibility for processing claims under the plan ended, for example after a plan changed administrators or a run-out period closed. The claim usually needs to go to the plan's current payer.

Quick facts

Code
CO-166 (CARC 166)
Status
Active In use since February 28, 2005.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The payer will not process the claim and the amount is not billable to the patient from this payer's decision. Redirect to the correct payer.
  • OA (Other Adjustment): Sometimes used because the payer is simply no longer the processor, without assigning responsibility to either party.
Official description
These services were submitted after this payers responsibility for processing claims under this plan ended.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-166 means

CARC 166 reads “These services were submitted after this payers responsibility for processing claims under this plan ended.” The payer was once responsible for the plan, but by the time your claim arrived, it no longer was. The patient may still have coverage; it is just handled by someone else now.

This commonly happens when an employer changes third-party administrators, a Medicaid program shifts members between managed care organizations, a Medicare contractor jurisdiction changes, or a plan terminates and its run-out period closes.

Example: an employer moves from one TPA to another on January 1. The old TPA processes claims for December services until March 31. A December claim sent to the old TPA in May is denied CO-166, and must go to the new administrator.

Common causes

  • Plan administrator change that the practice did not know about.
  • Run-out period closed before a late or resubmitted claim was received.
  • Medicaid managed care transitions where the member’s plan changed.
  • Outdated payer ID in the practice management system after a payer consolidation.
  • Resubmitting old claims (for example after a denial) to the original payer long after the transition.

How to fix it

  1. Confirm the current administrator with the patient, the employer, or an eligibility inquiry.
  2. Ask the old payer whether any exception applies, for example if you submitted originally within the run-out period.
  3. Submit the claim to the new payer with the correct payer ID and member ID in box 1a. This is a new claim to that payer, not a corrected claim.
  4. Attach or keep the CO-166 remittance as proof of timely good-faith filing if the new payer’s deadline is an issue.
  5. Update the patient’s insurance record so future claims route correctly.

How to prevent it

  • Verify eligibility before every visit and watch for payer ID or administrator changes. See eligibility denials.
  • Read payer transition notices from employers, Medicaid programs, and Medicare contractors.
  • Work old AR before run-out periods end, especially around plan-year changes.
  • Track CO-166 by payer to catch transitions affecting many patients at once.

Remark codes that may appear with CO-166

  • N418 (Misrouted claim.): The claim was misrouted to the wrong payer or processor.
  • PR-27 (Expenses incurred after coverage terminated.): The patient's coverage ended before the service date. CO-166 is about the payer's role ending, not the patient's coverage.
  • CO-29 (The time limit for filing has expired.): The claim was filed after the timely filing limit, another time-based denial.
  • CO-109 (Claim/service not covered by this payer/contractor.): The claim is not covered by this payer or contractor and should be sent to the correct one.
  • CO-31 (Patient cannot be identified as our insured.): The patient cannot be identified as the payer's insured.

CO-166 FAQ

What is a run-out period?

When an employer switches plan administrators or a plan terminates, the old payer usually keeps processing claims for services before the change for a limited period. Claims received after that run-out period may be denied with CO-166.

Who should I bill after CO-166?

Ask the patient or employer who administers the plan now, and check eligibility with that payer. The new administrator may accept claims for older dates of service, sometimes with its own filing deadline.

Does CO-166 count against timely filing?

The time spent with the wrong payer usually does not stop the new payer's clock automatically. Keep the CO-166 remittance as proof in case you need to request a timely filing exception.

Does CO-166 mean the patient has no insurance?

Not necessarily. It means this payer is no longer processing claims for the plan. The patient may be fully covered through a new administrator, so check before billing the patient.