Denial Code Decoder
CO-286 — The appeal arrived too late
The appeal was filed after the payer's appeal window closed. This is the code billers most often confuse with a timely filing denial, and the confusion is expensive: CO-29 is about when the CLAIM was submitted, measured from the date of service, while this one is about when the APPEAL was submitted, measured from the determination being appealed. Two clocks, two start dates, and only one of them is running once a denial lands.
Billable to the patient? No — a missed appeal deadline is a provider-side failure, and moving that balance to the patient is not defensible under most contracts.
Replaced code 138
Late appeals used to arrive as code 138, which also covered appeal procedures that were not followed. The replacement covers the deadline alone, so a procedural refusal now arrives under a different code and is worked differently. X12 stopped code 138 on May 1, 2018.
Where this denial is born
- Front desk
- Coding
- Claim build
- Submission
- Adjudication
This denial is usually created at the highlighted stage — that is where prevention lives.
Answer the questions — follow the path
The same questions an experienced biller asks, in order. Your answers draw the route to the right action.
Was the appeal genuinely filed after the payer's window closed?
Find the payer's window in the contract or the denial notice, and count from the determination date it names — not from the date of service.
The full decision tree
Was the appeal genuinely filed after the payer's window closed?
Find the payer's window in the contract or the denial notice, and count from the determination date it names — not from the date of service.
- Yes →
Is there a good-cause reason the deadline should be excused?
A payer's own error, a notice that never arrived, or records the payer held are the arguments that get heard. 'We were busy' is not one.
- Yes →
File a good-cause request. Document the circumstance with dates and correspondence, and ask for the deadline to be excused under the payer's own exception. Medicare has such a provision; commercial plans vary, so cite the plan's language, not Medicare's.
- No →
Close it — and find the queue that ate the clock. A genuinely late appeal with no exception is over. The value left is diagnostic: this denial waited somewhere, and whatever held it is holding others right now.
- Yes →
- No →
Answer with the filing evidence. Send the submission confirmation, portal receipt or delivery record showing the appeal arrived inside the window, and ask for the appeal to be reopened on that basis.
Prevention
Put the appeal clock on the denial the day it posts, not the day someone picks it up. The window runs from the determination, so every day a denial sits unworked in a queue is a day already spent.
Related reading
The code list, and when it was last checked
X12 maintains the Claim Adjustment Reason Codes list, where this code is listed as 286. The list carries the number alone: the CO in front of it is the adjustment group the payer chose for this line, which decides who owes the balance, and the same code can arrive under a different group from a different payer. It has been on the list since November 1, 2017.
Checked against the list revision dated November 1, 2025 on August 7, 2026, where it reads as an active code.
Sources
The paragraphs behind the guidance above. A page that tells you what to do about money should show you what the instruction rests on.
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