Noritz 90

Noritz error code 90: combustion abnormality

By USA Expert Plumbing Editorial Team. Updated September 29, 2026. 1 min read.

Short answer

Code 90 indicates a combustion abnormality caused by an improper mix of air and fuel. Noritz lists insufficient gas supply, a blockage in the vent and debris in the fan motor or unit as causes. Code 10 is often an early warning before code 90.

Code 90 is a combustion code. The heater detected that the mixture of air and gas was not right after repeated attempts, and it shut down.

Noritz notes that code 10, also a combustion abnormality, is usually a precursor to the more severe code 90.

If you smell gas, do not relight or reset anything and do not touch electrical switches. Leave the building and call your gas utility from outside.

Common causes

Insufficient gas pressure. An undersized line or a supply problem.
Venting blockage. An obstructed intake or exhaust, or a vent that is too long.
A dirty unit. Dust and lint inside the heater restrict combustion air. Noritz technicians clean units with compressed air.

What to do

Check the vent terminals for obvious blockages, then call a licensed contractor. The fix involves measuring gas pressure with a digital manometer, checking vent length and settings, and cleaning the inside of the unit.

Common questions

Can I reset Noritz code 90 myself?

Some models require a specific reset procedure after code 90. More importantly, the combustion problem needs to be diagnosed by a licensed contractor.

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Other Noritz codes

Code	Meaning
10	Combustion abnormality (warning)
11	Ignition failure
12	Flame loss
13	Optional CO alarm device fault
14	Thermal fuse blown (severe overheat)
16	Outlet water temperature spiked too high
20	High limit switch tripped
29	Condensate drain problem (condensing models)
67	Air gas mixer (AGM) abnormality
71	Gas valve circuit abnormality
90	Combustion abnormality
92	Neutralizer near end of life (first warning)
93	Neutralizer at end of life

All Noritz error codes