Free tools to understand denial codes and fix claims faster.

CO-16 means the payer couldn't process the claim because something on it was missing, incomplete, or incorrect. It's almost always paired with a second code (a RARC, usually starting with "N" or "M") that names the specific missing piece — the fix depends on reading that second code, not CO-16 alone.

CO-18 means the payer's system sees this as a duplicate of a claim or service line it already processed. That doesn't always mean it's actually a duplicate — sometimes two legitimate services just look identical to the payer's matching logic.

CO-197 means the payer required prior authorization, precertification, or notification before this service was performed, and their records show it either wasn't obtained or doesn't match what was billed.

CO-29 means the payer says the claim reached them after their filing deadline. Deadlines differ by payer and by contract, so the question is whether you can prove the claim was received in time, or that the delay had a reason the payer accepts.

CO-50 means the payer decided the billed service isn't medically necessary under their coverage rules — not that the care itself was wrong, just that it didn't meet the payer's documented criteria for this diagnosis on this date.