CO P21 denial code

Claim adjustment reason code P21. Often reported as CO P21 or OA P21.

Denied under auto PIP or MPC rules

Unofficial reference. This is our own plain-English explanation, not the official code text. Payer rules differ. Verify with the payer and the official X12 code list. AlphaPrefix is not affiliated with X12, CMS or any payer. Data last updated Nov 11, 2026.

What P21 means

The auto insurer denied payment based on personal injury protection (PIP) or medical payments coverage (MPC) rules or policy.

Only auto insurers use it, on medical claims under an auto policy. It means the auto carrier applied a regulation or policy for PIP or medical payments coverage and decided not to pay.

The remittance may point to the policy or rule involved, at the claim or line level. Common next payers include the patient's health plan or another auto policy.

Group codes seen with P21

On a remittance, P21 comes after a group code. These are the ones it is most often reported with:

• CO P21 (Contractual Obligation): The provider's side of the ledger. The reduction comes from the payer contract or a rule the provider has to follow, and the patient normally cannot be billed for it.
• OA P21 (Other Adjustment): Used for adjustments that are neither the provider's contractual write-off nor the patient's share. It often appears with coordination of benefits, for example when another payer already paid.

The payer chooses the group code for each adjustment, so the same reason code can appear with another group code on your remittance. Read the two together. More about group codes.

Common causes

• The injury is not covered under the PIP or medical payments policy terms.
• Required forms or notices were not filed in time under the state's auto rules.
• The patient was not an insured person under the policy.

What to do next

1. Ask the auto adjuster for the specific rule or policy reason.
2. Check whether the patient has other auto or health coverage to bill.
3. Correct missing forms or appeal under the state's auto claim process.

Related codes

• P22 (reason code) Adjusted under auto PIP or MPC rules
• P23 (reason code) Auto PIP or MPC fee schedule adjustment
• P27 (reason code) Denied under auto liability rules