CO 163 denial code

Claim adjustment reason code 163. Often reported as CO 163.

Referenced attachment not received

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 163 means

The claim said documentation was being sent, but the payer never received the attachment.

Your claim pointed to an attachment, such as records, an invoice or a form, and the payer has nothing on file for it. Without it the payer could not finish processing.

Check how and when the attachment was sent and whether the attachment control number on the claim matched the one on the paperwork.

Group codes seen with 163

On a remittance, 163 comes after a group code. These are the ones it is most often reported with:

• CO 163 (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 attachment was never sent after the claim went out.
• The attachment control number on the claim did not match the one on the paperwork.
• The documents were sent to the wrong address, fax number or portal.

What to do next

1. Confirm whether and how the attachment was sent, and check the control number.
2. Send the documents again using the payer's attachment method with the matching control number.
3. Ask the payer to reprocess once it confirms the attachment is on file.

Related codes

• 164 (reason code) Attachment received too late
• 16 (reason code) Missing information or billing error
• 252 (reason code) Attachment or documentation required