CO 164 denial code

Claim adjustment reason code 164. Often reported as CO 164.

Attachment received too late

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 164 means

The payer received the documentation the claim referred to, but after its deadline, so it was not used.

The attachment did arrive, but past the time the payer allows after a claim is filed. The payer processed or denied the claim without it.

Payers set their own deadlines for attachments. Check the payer's policy and whether the claim can be reopened or appealed with the documents.

Group codes seen with 164

On a remittance, 164 comes after a group code. These are the ones it is most often reported with:

• CO 164 (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 documents were sent well after the claim.
• The attachment was delayed by a mail, fax or portal problem.
• The claim was filed before the documentation was ready to send.

What to do next

1. Check the payer's deadline for attachments and when yours arrived.
2. Appeal or request reopening with the documents and proof of when they were sent.
3. Set up a process so attachments go out at the same time as the claim.

Related codes

• 163 (reason code) Referenced attachment not received
• 29 (reason code) Timely filing limit has passed