CO 16 denial code

Claim adjustment reason code 16. Often reported as CO 16.

Missing information or billing error

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 16 means

The payer could not finish processing because the claim was missing data or had a submission or billing error.

Think of this code as a pointer, not the full answer. It always comes with at least one remark code (or a pharmacy reject code) that tells you exactly what was missing or wrong. Read the remark code before changing anything.

It is meant for missing or invalid claim data, not for requests for medical records or other attachments. Most payers treat it as a rejection you can fix with a corrected or new claim rather than an appeal. It is usually CO, so do not move the balance to the patient.

Group codes seen with 16

On a remittance, 16 comes after a group code. These are the ones it is most often reported with:

• CO 16 (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

• A required field was blank or invalid, such as a referring provider, NPI, taxonomy or date.
• Subscriber or patient details on the claim did not match the payer's eligibility file.
• A required modifier, unit count, NDC or other line level detail was missing or wrong.
• Coordination of benefits information, such as the other payer's details, was missing.
• The claim was formatted or sent in a way the payer's system could not read.

What to do next

1. Read every remark code on the line and the claim; they name the actual problem.
2. Fix the specific data element in the practice management system so the next claim is right too.
3. Send a corrected or new claim as the payer requires rather than filing an appeal.
4. If the remark code is unclear, call the payer or check its portal for the claim detail before resubmitting.
5. Track repeat causes and add a scrubber edit to stop the same error going out again.

Related codes

• N382 (remark code) Patient ID missing or invalid
• MA130 (remark code) Unprocessable claim: correct and resubmit
• 4 (reason code) Procedure code and modifier do not match
• 29 (reason code) Timely filing limit has passed