Noritz NRC Series Code 91
Improper gas type. The appliance has decided it is being supplied with the wrong gas. Noritz's diagnosis column is three words — check gas type — and the remark is that power has to be cycled to clear it. A natural gas unit fed propane, or a conversion done without the correct kit, burns dangerously rich.
Call a proThe verdict
Call a licensed plumber
Leave it off. This is the one code in the table where the appliance is telling you the fuel itself is wrong, and the failure mode is carbon monoxide rather than no hot water. Find out whether the unit was ever converted, and get it checked before it runs again.
Typical cost: $150–$900
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the NRC Series — not averaged across the brand.
Source: Noritz Service Manual — NR501-OD, NR662-OD, NRC661, NRC711 (Rev. Dec 2013) (p. 28), read Sep 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Switch the unit off and leave it off.
- Find out whether the heater was ever converted between natural gas and propane, and whether the correct Noritz kit was used.
- Check the rating label on the unit against what your supply actually is — they should match.
- Call a licensed professional before running it again, and make sure the house has a working carbon monoxide alarm.
Frequently asked questions
The unit has run fine for years. How can the gas type suddenly be wrong?
The gas usually has not changed — what changed is the appliance's ability to burn it cleanly, and the symptom looks the same to the control board. A partly blocked air path, a drifting gas valve or a fan running slow all move combustion in the same direction. That is why this code is a professional diagnosis rather than a note to your gas supplier.