CO B4 denial code

Claim adjustment reason code B4. Often reported as CO B4.

Late filing penalty

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 B4 means

The payer paid the claim but took a penalty reduction because it was filed later than the payer's rules allow.

Unlike a full timely filing denial, B4 means the claim was still processed but the payment was cut as a penalty for late submission. Some payers and programs use a reduction rather than a flat denial for claims that arrive late.

The penalty is usually treated as the provider's loss. To get it back, you generally need proof that the claim was submitted on time or that a valid reason for delay applies.

Group codes seen with B4

On a remittance, B4 comes after a group code. These are the ones it is most often reported with:

• CO B4 (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 first claim was sent after the payer's filing deadline.
• An earlier submission was rejected at the clearinghouse and never reached the payer.
• A corrected or secondary claim was filed late after the primary payer's decision.

What to do next

1. Look up the original submission date and any clearinghouse acceptance reports for the claim.
2. Appeal with proof of timely submission if the claim was sent on time.
3. Check the payer's policy for accepted reasons for late filing, such as retroactive eligibility.
4. Review your claim submission and rejection workflows to stop late filings from recurring.

Related codes

• 29 (reason code) Timely filing limit has passed
• N892 (remark code) Delay reason code not accepted