Denial Code Decoder
M77 — Place of service is wrong or missing
The place-of-service code on the line is absent, invalid, or not one the payer will accept for the service billed. It is the most concrete remark code in this set, because it names a single field you can go and look at — and it is worth knowing that CMS instructs its contractors to return a claim carrying more than one place of service on a single Part B claim, paired with CO-16, which is exactly this combination.
Billable to the patient? No — a defective field on your own claim is a provider-side correction.
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.
Does the claim carry exactly one place-of-service code?
Two failures answer no here and they are fixed differently: the field can be empty, or the claim can carry more than one setting — which is itself a defect, because those services belong on separate claims.
The full decision tree
Does the claim carry exactly one place-of-service code?
Two failures answer no here and they are fixed differently: the field can be empty, or the claim can carry more than one setting — which is itself a defect, because those services belong on separate claims.
- Yes →
Does it match where the service was actually furnished?
Check the encounter, not the template default. Telehealth, hospital outpatient and office are the three that get confused most.
- Yes →
Check what this payer accepts for this service. If the code is right for where the work happened, the payer may not accept that setting for that service — a payer-policy question, not a data one. Read the policy before you correct anything.
- No →
Correct the code and resubmit. Report the setting the record supports, then resubmit as a replacement claim referencing the original claim number.
- Yes →
- No →
Supply the code, or split the claim by setting. If the field was simply empty, report the setting the record supports and resubmit as a replacement. If the claim carried more than one setting, submit one claim per place of service instead, each with the address of where those services were furnished.
Prevention
Derive the place of service from where the service was actually furnished rather than from a default on the encounter template, and split the claim when a patient was seen in two settings on the same day.
Related reading
The code list, and when it was last checked
X12 maintains the Remittance Advice Remark Codes list, where this code is listed as M77. A remark code carries no adjustment group of its own — the group you see on the remittance line belongs to the reason code it arrived with. It has been on the list since January 1, 1997, and X12 last revised its entry on March 14, 2014.
Checked against the list revision dated July 1, 2026 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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