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M20 Remark Code: Missing or Invalid HCPCS Code

M20 means the claim line has a HCPCS code that is missing, incomplete, or invalid, so the payer could not process it. The code must be corrected before the line can be paid.

Quick facts

Code
M20 (RARC M20)
Status
Active In use since January 1, 1997; last modified February 28, 2003.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The line is unpaid until a valid code is submitted. It is a provider issue and not billable to the patient.
Official description
Missing/incomplete/invalid HCPCS.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What M20 means

HCPCS codes identify what was provided: Level I covers physician procedures and Level II covers supplies, drugs, equipment, ambulance, and other items. M20 tells you the HCPCS code on this line did not pass the payer’s validation. It may be blank, truncated, formatted wrongly, or not active on the date of service.

It usually arrives with CARC 16 (missing or invalid information) or CARC 181 (code invalid on date of service), and many payers treat it as unprocessable.

Common causes

  • A deleted or not-yet-effective code was used for the date of service.
  • Typographical errors, such as a transposed digit or a letter O instead of zero.
  • A placeholder or internal charge code was sent instead of a HCPCS code.
  • A code that does not exist for this type of claim was used, for example a Level II code the payer does not recognize.
  • The field was empty because a charge was not mapped in the billing system.

How to fix it

  1. Look up the code in the current HCPCS files for the date of service.
  2. Replace it with the correct, active code and matching modifiers in box 24D.
  3. Resubmit. If the claim was returned as unprocessable, submit it as new; if processed, send a corrected claim with resubmission code 7.
  4. Fix the source in your chargemaster so the error does not repeat.

How to prevent it

Load HCPCS updates into your chargemaster every quarter and retire deleted codes promptly. A pre-submission scrub such as the Claims Validator can catch inactive codes before they reach the payer. See claim rejection vs denial for how unprocessable claims differ from denials.

Codes that may appear with M20

  • CO-16 (Claim/service lacks information or has submission/billing error(s).): The claim lacks information or has billing errors, here an invalid code.
  • CO-181 (Procedure code was invalid on the date of service.): The procedure code was invalid on the date of service.
  • CO-182 (Procedure modifier was invalid on the date of service.): The procedure modifier was invalid on the date of service.
  • M51 (Missing/incomplete/invalid procedure code(s).): Missing or invalid procedure code in general, not specific to HCPCS.
  • N56 (Procedure code billed is not correct/valid for the services billed or the date of service billed.): The code is valid but not correct for the service or date billed.
  • N65 (Procedure code or procedure rate count cannot be determined, or was not on file, for the date of service/provider.): The procedure code rate could not be determined or was not on file.

M20 FAQ

What is the difference between M20 and M51?

M20 refers specifically to a HCPCS code; M51 covers procedure codes generally. Payers pick one based on the field involved, but the fix is the same: submit a valid code.

Why would a code that worked last year fail now?

HCPCS Level II codes are added, revised, and deleted on a regular schedule. A code deleted before the date of service will be rejected.

Should I appeal M20?

No. It is a data error. Submit a corrected or new claim with the valid code.