CO 166 denial code

Claim adjustment reason code 166. Often reported as CO 166.

Billed after payer's responsibility ended

Unofficial reference. This is our own plain-English explanation, not the official code text. Payer rules differ. Verify with the payer and the official X12 code list. AlphaPrefix is not affiliated with X12, CMS or any payer. Data last updated Nov 11, 2026.

What 166 means

The payer no longer processes claims for this plan, because its responsibility for it ended before the claim was sent.

The payer used to handle this plan, but its processing role has ended, for example after a contract change, takeover or run-out period. Claims sent after that are not handled.

This usually means the claim needs to go to a different payer or administrator that now handles the plan. The patient's current card or the payer's notice should tell you who.

Group codes seen with 166

On a remittance, 166 comes after a group code. These are the ones it is most often reported with:

• CO 166 (Contractual Obligation): The provider's side of the ledger. The reduction comes from the payer contract or a rule the provider has to follow, and the patient normally cannot be billed for it.

The payer chooses the group code for each adjustment, so the same reason code can appear with another group code on your remittance. Read the two together. More about group codes.

Common causes

• The employer or plan moved to a new administrator.
• The payer's run-out period for the plan closed before the claim was filed.
• Your system still lists the old payer for this patient.

What to do next

1. Ask the patient or the payer who handles claims for this plan now.
2. Update the payer on the patient's account.
3. Send the claim to the correct payer with proof of the original filing date if timely filing could be an issue.

Related codes

• 109 (reason code) Not covered by this payer, send elsewhere
• 29 (reason code) Timely filing limit has passed