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N166 Remark Code (Deactivated): Mileage Without Patient

N166 meant payment for mileage was denied or reduced because the patient was not in the ambulance for those miles. X12 deactivated it and suggested N159, which carries the same text.

X12 deactivated RARCN166 on January 31, 2004. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: N166 (RARC N166)
Status: Deactivated StoppedJanuary 31, 2004 (in use since February 28, 2003).
Code set: Remittance Advice Remark Codes (RARC)
Group codes: CO (Contractual Obligation): The denied mileage was an ambulance supplier write-off.
Official description: Payment denied/reduced because mileage is not covered when the patient is not in the ambulance.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What N166 meant

Ambulance claims bill a base rate plus mileage. Remark N166 applied when the mileage billed included miles traveled without the patient aboard, for example driving to the scene or back to the station. Payers pay mileage only while the patient is being transported, so the extra miles were denied or cut.

What replaced it

X12’s note suggested N159. N159 carries the same message and is active. Other ambulance-related remarks include N157 for non-covered destinations and N745 when the ambulance report is missing.

If you still see N166

It was short-lived and appears only in older data. For current mileage denials, compare billed miles with the trip record from pickup to destination, correct the units, and resubmit if the original claim overstated loaded mileage.

N159 (Payment denied/reduced because mileage is not covered when the patient is not in the ambulance.): The suggested replacement, with the same wording.
N157 (Transportation to/from this destination is not covered.): Transportation to or from this destination is not covered.
N745 (Missing Ambulance Report.): Missing ambulance report.

N166 FAQ

Which mileage is payable?

Generally only loaded mileage, meaning miles traveled with the patient on board. Miles driven to reach the patient or return to base are not billed as patient mileage.

How can this denial be avoided?

Report only loaded miles, and make sure the trip documentation shows the pickup and drop-off points and odometer readings or equivalent.

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