What causes Noritz Tankless Water Heater 90?

Dust, lint or oil clogging the air intake: the front cover intake, the internal air filter screen, the intake screen or elbow on top of the unit, or the outdoor intake termination. A blocked exhaust vent or exhaust termination (debris, nests, snow or soot). A clogged or frozen condensate drain line on condensing models. Low gas supply pressure, causing abnormal combustion. On new installs: incorrect or poorly installed venting, or dip switches not set for elevation or vent pipe size and length. Build-up inside the unit on the burner, fan or heat exchanger fins, usually from long use in a dusty or dirty space without regular servicing.

How do I fix Noritz Tankless Water Heater 90?

If you smell gas, stop, leave the house and call your gas utility from outside. Turn the heater off with its power button and let it cool before touching anything, as Noritz's owner's guide warns. If the heater takes combustion air from the room, look at the air supply vent and the intake openings on the front cover and top of the unit. Noritz's owner's guide says to remove dust, oil or other build-up with a vacuum cleaner or damp towel, but not to fully remove the inlet screen or air screen. Go outside and check the intake and exhaust vent terminations on the roof or sidewall. Clear away leaves, nests, snow, ice or anything else blocking them. On a condensing model, check that the condensate drain line isn't clogged or frozen where it discharges. Clear the code by turning the heater off and back on, then run hot water. If the code doesn't come back, the problem is solved. If the heater is newly installed, or the code returns after cleaning, call the installer, a licensed plumber or gas technician, or Noritz technical support at 866-766-7489. Gas pressure, venting and dip-switch settings need a professional.

When should I call a pro for Noritz Tankless Water Heater 90?

Using the maintenance monitors through the remote controller with the heater running and the front cover on: Noritz says MM31 (outlet temperature) should read within about 2 degrees of the set temperature, and MM49 (fan speed) should read 100. If MM31 is well below set temperature: diagnosing why not enough gas is reaching the burner, inside or outside the unit, including measuring gas supply pressure. If MM49 reads above 100 (for example 105, 107 or 110): finding what is restricting intake air or exhaust, from the air filter and intake screens to the vent runs and terminations. Checking that venting matches Noritz's installation requirements and that the dip switches are set for the elevation and vent pipe size and length. Removing and cleaning the fan, burner and heat exchanger fins when they are packed with lint or soot, which Noritz says requires opening the gas system and should be done by a professional. Clearing a clogged or frozen condensate drain line, and contacting Noritz with the model and serial number if code 90 continues.

Code: 90🚿 Noritz Tankless Water Heater

Noritz Error Code 90: Combustion Abnormality (Meaning, Causes, Fixes)

Q: What does Noritz Tankless Water Heater 90 mean?

Code 90 on a Noritz tankless water heater means a combustion abnormality: the fuel-and-air mix is off, so the heater is either struggling to move air in or out or can't reach the set temperature. On an older unit the most common cause is dust or lint clogging the air intake, which you can check and clean yourself. On a new install it's more often low gas pressure, venting, or dip-switch settings, which belong to the installer. Noritz's knowledge base describes error code 90 as a combustion abnormality: the mixture of fuel and air is off specification and the heater is burning poorly. Put simply, the unit is either struggling to pull in combustion air or push out exhaust, or it can't reach the set temperature. When airflow is restricted, the heater speeds its fan above normal to compensate, and because that is an abnormal condition it raises code 90. Noritz's owner's guide lists codes 90, 901 and 902 together, with four possible causes: a clogged air supply vent (on units that take combustion air from indoors), a clogged exhaust vent, abnormal combustion from low gas supply pressure, or a clogged or frozen condensate drain line. Noritz says that on a newly installed heater, code 90 usually points to insufficient gas pressure, incorrect or poorly installed venting, or dip switches not set for the elevation or vent pipe size and length. On a heater that has been running for a while and only recently started showing 90, the more likely cause is a dirty unit from lack of maintenance. The knowledge base lists it as applying to all Noritz models.

Reviewed 2026-10-23 · Based on 2 manufacturer & service sources

What the code means

Noritz's knowledge base describes error code 90 as a combustion abnormality: the mixture of fuel and air is off specification and the heater is burning poorly. Put simply, the unit is either struggling to pull in combustion air or push out exhaust, or it can't reach the set temperature. When airflow is restricted, the heater speeds its fan above normal to compensate, and because that is an abnormal condition it raises code 90. Noritz's owner's guide lists codes 90, 901 and 902 together, with four possible causes: a clogged air supply vent (on units that take combustion air from indoors), a clogged exhaust vent, abnormal combustion from low gas supply pressure, or a clogged or frozen condensate drain line. Noritz says that on a newly installed heater, code 90 usually points to insufficient gas pressure, incorrect or poorly installed venting, or dip switches not set for the elevation or vent pipe size and length. On a heater that has been running for a while and only recently started showing 90, the more likely cause is a dirty unit from lack of maintenance. The knowledge base lists it as applying to all Noritz models.

Likely causes

Dust, lint or oil clogging the air intake: the front cover intake, the internal air filter screen, the intake screen or elbow on top of the unit, or the outdoor intake termination
A blocked exhaust vent or exhaust termination (debris, nests, snow or soot)
A clogged or frozen condensate drain line on condensing models
Low gas supply pressure, causing abnormal combustion
On new installs: incorrect or poorly installed venting, or dip switches not set for elevation or vent pipe size and length
Build-up inside the unit on the burner, fan or heat exchanger fins, usually from long use in a dusty or dirty space without regular servicing

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

If you smell gas, stop, leave the house and call your gas utility from outside.
Turn the heater off with its power button and let it cool before touching anything, as Noritz's owner's guide warns.
If the heater takes combustion air from the room, look at the air supply vent and the intake openings on the front cover and top of the unit. Noritz's owner's guide says to remove dust, oil or other build-up with a vacuum cleaner or damp towel, but not to fully remove the inlet screen or air screen.
Go outside and check the intake and exhaust vent terminations on the roof or sidewall. Clear away leaves, nests, snow, ice or anything else blocking them.
On a condensing model, check that the condensate drain line isn't clogged or frozen where it discharges.
Clear the code by turning the heater off and back on, then run hot water. If the code doesn't come back, the problem is solved.
If the heater is newly installed, or the code returns after cleaning, call the installer, a licensed plumber or gas technician, or Noritz technical support at 866-766-7489. Gas pressure, venting and dip-switch settings need a professional.

🛠 When to call a pro

Using the maintenance monitors through the remote controller with the heater running and the front cover on: Noritz says MM31 (outlet temperature) should read within about 2 degrees of the set temperature, and MM49 (fan speed) should read 100.
If MM31 is well below set temperature: diagnosing why not enough gas is reaching the burner, inside or outside the unit, including measuring gas supply pressure.
If MM49 reads above 100 (for example 105, 107 or 110): finding what is restricting intake air or exhaust, from the air filter and intake screens to the vent runs and terminations.
Checking that venting matches Noritz's installation requirements and that the dip switches are set for the elevation and vent pipe size and length.
Removing and cleaning the fan, burner and heat exchanger fins when they are packed with lint or soot, which Noritz says requires opening the gas system and should be done by a professional.
Clearing a clogged or frozen condensate drain line, and contacting Noritz with the model and serial number if code 90 continues.

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.

Noritz Error Code 90: Combustion Abnormality (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Related codes

Sources