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CO-21 Denial Code: No-Fault Carrier Is Liable

CO-21 means the payer determined the injury or illness is the liability of a no-fault carrier, typically personal injury protection (PIP) auto insurance. Bill the no-fault carrier first; the health plan generally considers the claim only after no-fault benefits are denied or exhausted.

Quick facts

Code
CO-21 (CARC 21)
Status
Active In use since January 1, 1995; last modified September 30, 2007.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The payer's common group. Don't bill the patient; bill the no-fault carrier or show the plan that no-fault benefits don't apply.
  • OA (Other Adjustment): Used by some payers to indicate another carrier is responsible, with no amount assigned to the provider or patient.
  • PR (Patient Responsibility): Sometimes assigned to the patient. No-fault states often have rules about billing patients for covered accident care; check them first.
Official description
This injury/illness is the liability of the no-fault carrier.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-21 means

CARC 21 says this injury or illness is the liability of the no-fault carrier. In states with no-fault auto insurance, and wherever a patient’s auto policy includes personal injury protection or medical payments coverage, the auto insurer often pays first for accident injuries regardless of fault. The health plan uses CO-21 to say that carrier, not the plan, should be billed first.

The difference from CO-20 matters when you redirect the claim. Liability coverage belongs to the at-fault party and may not pay until a claim is settled. No-fault coverage usually belongs to the patient’s own auto policy (or the vehicle they were in) and pays under its own rules and limits, often with state-specific forms and deadlines.

Common causes

  • Box 10b (auto accident) marked yes, with a state code, on the claim.
  • The patient reported an auto accident to the health plan.
  • A no-fault insurer reported ongoing responsibility for the diagnosis (N727).
  • Intake didn’t capture the auto policy and PIP claim number.

How to fix it

  1. Confirm the accident and whether the patient has PIP or medical payments coverage, through their own policy or the vehicle they were in.
  2. Collect the carrier, claim number, and adjuster from the patient or their auto insurer.
  3. Bill the no-fault carrier using its required forms and state rules, which may include specific deadlines and reports.
  4. When no-fault benefits are exhausted or denied, submit the claim to the health plan with the no-fault EOB or denial letter.
  5. If there was no auto accident, correct box 10b and send a corrected claim with resubmission code 7 in box 22.
  6. Hold off on billing the patient until you know the no-fault and health plan outcome and have checked state rules.

How to prevent it

  • Ask every injury patient about auto accidents and capture the auto policy details at the first visit.
  • Create a separate accident case in your system so claims route to the no-fault carrier first.
  • Track PIP limits and exhaustion so you know when to switch to the health plan.
  • Keep copies of no-fault EOBs and denials ready for the health plan. See eligibility and COB denials.

Specialty notes

Chiropractors, PT practices, and orthopedic groups treat many auto injuries and see CO-21 frequently. In some states, no-fault carriers have their own fee schedules and require specific forms, so the practice’s accident workflow needs to follow state rules closely.

Remark codes that may appear with CO-21

  • N727 (A no-fault insurer has reported having ongoing responsibility for medical services (ORM) for this diagnosis.): A no-fault insurer has reported ongoing responsibility for medical services for this diagnosis.
  • MA92 (Missing plan information for other insurance.): The claim lacks plan information for the other insurance, here the no-fault carrier.
  • CO-20 (This injury/illness is covered by the liability carrier.): A liability carrier (at-fault party's insurer) is responsible rather than a no-fault carrier.
  • OA-19 (This is a work-related injury/illness and thus the liability of the Worker's Compensation Carrier.): Workers' compensation is responsible for a work injury.
  • CO-P21 (Payment denied based on the Medical Payments Coverage (MPC) and/or Personal Injury Protection (PIP)): The no-fault or PIP carrier's own denial based on its jurisdictional rules, reported on P&C remittances.
  • OA-22 (This care may be covered by another payer per coordination of benefits.): General coordination of benefits: another payer may be primary.

CO-21 FAQ

What is a no-fault carrier?

An auto insurer that pays medical expenses for people injured in a car accident regardless of who caused it, usually through personal injury protection (PIP) or medical payments coverage. Requirements vary by state.

When will the health plan pay after CO-21?

Usually after the no-fault carrier pays its limit, denies the claim, or confirms the injury isn't covered. Send the no-fault EOB or denial to the health plan with the claim.

What if the patient wasn't in a car accident?

Correct box 10b and any related information, then send a corrected claim. The patient may also need to update the accident information with their plan.