CO-4 means the procedure code is inconsistent with the modifier billed, or a required modifier is missing.
On behavioral health facility claims it usually shows up on HCPCS lines such as H0015, S9480 or H0035 when the payer's billing guide asks for a modifier that was not sent, or when a modifier from another payer's setup was reused.
Pull the payer's current billing guide and your contract, add or fix the modifier, and resubmit as a corrected claim. Then fix the charge master or billing rule so the next claim goes out right.
Questions
Do I appeal a CO-4?
No. Send a corrected claim with the right modifier.
Where do I find the modifier a payer wants?
In the payer's billing guide or reimbursement policy for the service, and in your contract.
Read next
Educational summary, not billing advice. Official code descriptions are at x12.org. Checked October 2026.