CO 29 Denial Code: Meaning, Reason, Example and Fixation
When a claim is not received within the Medicare timely filing limit, it doesn’t just get delayed—it may become non-payable unless a valid CMS exception applies. One of the most common questions providers ask is: What is CO 29 denial code in Medicare? The answer is simple: it means the claim was filed too late. Although CO 29 is commonly associated with Medicare timely filing denials, the code is also used by Medicaid and commercial payers for late-filed claims
In Medicare billing, the most common codes that indicate a missed deadline are CO 29 denial code (Part B) and N390 denial code (Part A). Both are triggered when a claim is received after Medicare’s 12-month timely filing rule.
Understanding CO 29 and N39011 Denial Codes
In Medicare billing, every denied claim includes standardized codes that explain why payment was refused. These include:
- Claim Adjustment Reason Codes (CARCs)
- Remittance Advice Remark Codes (RARCs)
Among these, CO 29 denial code and N390 specifically indicate a timely filing denial, meaning the claim was submitted after the allowed timeframe.
What is CO 29 Denial Code in Medicare?
The CO 29 denial code means:
“The time limit for filing claim has expired.” It is one of the most searched and most critical denial codes in medical billing and is directly tied to the Medicare timely filing limit of 12 months for professional claims.
When Does CO 29 Occur?
CO 29 is assigned when:
- A claim is received after 12 calendar months from the Date of Service (DOS)
- The filing deadline defined under Medicare timely filing rules has passed
Important: Medicare generally uses the claim receipt date recorded by the MAC, not the submission or postmark date.
Example:
- Date of Service: June 10, 2025
- Claim received by Medicare: June 15, 2026
Result: Denied with CO 29 denial code due to exceeding the 12-month filing limit
What Is N390 Timely Filing Remark Code?
Some MACs or remittance systems may display internal variations such as N39011, but the standardized RARC is N390. This code applies to institutional providers — hospitals, skilled nursing facilities (SNFs), and home health agencies — that file inpatient or facility-based claims. It is the Part A equivalent of CO 29 and may accompany claims with submission or processing issues, which indicate late or incomplete submissions.
Example:
- Discharge Date: January 2, 2025
- Claim submitted: February 5, 2026
Result: Denied with N39011 due to violation of the Medicare timely filing limit
Where CO-29 and N390 Denials Appear in the ERA (835)
| Code Type | Example Code | Appears In | Meaning |
|---|---|---|---|
| CARC | CO 29 | 835 Adjustment Segment | Claim filed after Medicare timely filing limit (Part B) |
| RARC | M290 / MA39 | 835 Remark Segment | Further explains why claim missed the filing window |
| Part A Reject Code | N390 | MAC Return / Remittance | Institutional claim exceeded 12-month rule |
Reasons of Denial CO 29 or N390 Codes
While CO 29 and N390 sound like technical codes, they often signal simple operational issues that went unnoticed until it was too late. In almost every case, the root cause is the same. Let’s break down the most common reasons this happens in 2025.
Claim Filed After the 12-Month Limit
The most direct cause. If the MAC receives the claim—even one day past the 12-month deadline—it automatically triggers CO 29 (Part B) or N390 (Part A).
Example: DOS = May 1 2024 → Received May 2 2025 = Denied.
Returned-to-Provider (RTP) or Rejected Claims Not Resubmitted in Time
Unprocessable (RTP) claims do not preserve the original submission date. If they are not corrected and resubmitted within the same 12-month period, Medicare treats them as new late filings.
Tip: Always correct and re-submit RTPs within 10 business days.
Incorrect or Delayed Patient Eligibility Updates
If Medicare coverage is added or corrected retroactively, any previously denied claim must still meet the timely filing limit—unless an approved retroactive eligibility exception is filed.
Example: A patient gains retroactive Part B entitlement six months after service; the provider must request a reopening (TOB xxQ) with eligibility proof if the 12-month window has passed.
MAC or Clearinghouse Rejection Not Followed Up
Sometimes the claim leaves the practice management system but never reaches Medicare because of a clearinghouse or EDI rejection. If the billing team doesn’t monitor daily acknowledgments (999/277CA files), the claim might remain unsubmitted until the window closes.
Provider Enrollment or PTAN Delays
If the rendering provider’s NPI/PTAN was not active at the time of claim submission, Medicare automatically rejects the claim. If credentialing takes too long, the filing deadline can expire before re-submission—producing CO 29 or N390.
Administrative or System Errors (MAC/CMS)
Rare but recognized by CMS. If a system outage or processing error prevented timely submission, providers may later file under the administrative error exception in IOM § 70.7 with documented proof.
Claim Lifecycle Path to Denial Code CO 29 or Code N390 Denial
DOS → Claim created → Submitted to clearinghouse
EDI Rejected (RTP) or Not Transmitted
No Resubmission within 12 Months
MAC Receives Late Claim
Denial Issued: CO 29 (Part B) or N390 (Part A)
Examples of CO 29 and N390 Denials
Example 1: Part B Outpatient Claim Denied with CO 29
Scenario of Claim denied with CO 29 Code
A physical therapist submits a claim for services rendered on June 1, 2024. Due to staff turnover, the claim is not transmitted through the clearinghouse until June 10, 2025.
Result
The MAC receives the claim 9 days past the 12-month window. The claim automatically denies with CO 29.
Resolution
Since no CMS exception applies, the claim cannot be appealed or reopened. The loss caused with CO 29 denial is final.
Example 2: Institutional Claim Denied with N390 (Part A)
Claim denied with N390 Scenario
A hospital discharges a patient on January 2, 2024, but delays its final bill while awaiting documentation. The claim is submitted on February 5, 2025.
Result
Because the submission occurred over 13 months after the “Through” date, the MAC issues denial code N390 – Timely filing expired.
Resolution
The hospital may only file a reopening (TOB xxQ) if it can prove a valid CMS exception, such as a system error or retroactive beneficiary entitlement. Otherwise, the denial stands final.
How to Fix CO 29 and N390 Denials
When you receive a CO 29 (Part B) or N390 (Part A) denial, it means your claim was filed after Medicare’s 12-month deadline and has been automatically disallowed for payment. However, under CMS IOM § 70.7, certain exceptions permit a reopening if specific conditions are met. Here’s exactly how to handle it.
Confirm the Denial Type and Claim Timeline
Check the ERA/835 remittance advice or MAC portal for the denial message:
- CO 29: “Time limit for filing claim has expired.” (Part B)
- N390: “Timely filing expired.” (Part A)
Then verify the Date of Service (DOS) or Through date against the claim’s receipt date. If the claim was received after 12 months, proceed only if a CMS exception applies.
Determine If a Valid CMS Exception Exists
Under CMS Pub 100-04 § 70.7, only a few circumstances qualify for a reopening:
| Exception | Proof Required |
|---|---|
| Administrative error by CMS or MAC | Written documentation of system or processing failure |
| Retroactive Medicare entitlement | SSA or CMS entitlement letter showing effective date |
| Retroactive coverage or enrollment | Proof of coverage correction issued after DOS |
| Natural disaster or public emergency | FEMA or MAC-issued disaster notice |
If none of these apply, the denial is final and unpayable.
File a Reopening Request (TOB xxQ)
A reopening is not the same as a standard appeal — it’s a special process used only for timely filing exceptions.
How to submit
- Identify the denied claim number and DOS.
- Institutional providers may need to submit an adjustment or reopening request using the appropriate TOB format required by their MAC
- Attach supporting documentation proving the exception (e.g., eligibility letter, MAC memo, disaster declaration).
- Submit via MAC portal, EDI, or paper as instructed on your MAC’s website.
- Retain confirmation of submission for audit purposes.
Example
A retroactive entitlement letter dated March 2025 shows Part B coverage effective April 2024. The claim for April 2024 services denied with CO 29 can be reopened with this proof.
Track the Reopening Status
MACs generally process reopening requests within 30–60 days. You can monitor progress through the MAC’s provider portal or inquiry system (e.g., Noridian Direct Data Entry, CGS Portal, FCSO SPOT, etc.).
If the reopening is approved, payment is reissued. If denied, the MAC will cite the reason (e.g., insufficient documentation, invalid exception).
Conclusion
CO 29 and N390 denials are not routine billing issues—they are final outcomes of missed claim deadlines. Once a claim falls outside the allowed timeframe, it is no longer payable unless a valid exception can be proven.
For providers, this means every delayed submission carries real financial consequences. Even a small number of late claims can quickly turn into unrecoverable revenue loss and operational inefficiencies.
The key to avoiding these denials is not reactive correction, but proactive claim management. Timely submission, consistent follow-up on rejected claims, and clear visibility into claim status are essential to keeping your revenue cycle on track.
At MedStates, we work closely with providers to identify gaps in their billing workflows, strengthen submission timelines, and support reopening cases where applicable. Our focus is simple: reduce preventable denials and protect your reimbursements.
If your practice is experiencing repeated CO 29 or N39011 denials, it may be time to evaluate your current processes and implement a more structured approach to claim management.
Contact MedStates today to safeguard your reimbursements and maintain full compliance with 2026 CMS timely filing rules
