Noritz NR98 & NR111 Code 90
Combustion fault. Combustion abnormality, in two grades under one number. The warning indication checks abnormal combustion and the flue for blockage; the shutdown adds checking that the flame ignites all the way across the burner and requires a power cycle to clear. Noritz splits combustion faults on this table into 90 and 99 and gives them the same checks, where the NRC661 range spreads the same ground across 90, 901, 902, 903 and 908. Fewer numbers here means less to go on, not fewer causes.
Also displayed as: F90
Call a proThe verdict
Call a licensed plumber
Check the vent terminal outside, because it is free and it is a common cause. Everything past it is combustion, and a unit reporting abnormal combustion indoors is one to leave off rather than keep restarting.
Typical cost: $0–$1,000
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 NR98 & NR111 — not averaged across the brand.
Source: Noritz Service Manual — NR50 / NR66 / NR71 / NR98 / NCC199 / NRC111 / NR111 / NC380 (Rev. July 2011) (p. 56), 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.
- Go outside and check the vent terminal and air inlet for snow, leaves, a nest, or anything built against them.
- Do not take any vent section apart yourself.
- Call a licensed professional.
Code 90 on other Noritz series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit: