Error Code 90

EC90 or Error Code 90

A 90 error code indicates a combustion abnormality. This means that the mixture of fuel and air is off from specification and is resulting in poor quality combustion.

To simplify it, the unit is either struggling to move air in or out of the unit or it is unable to hit the set temperature.

On a newly installed tankless unit giving a 90 error code, the problem more often lies in insufficient gas pressure to the unit, incorrect or poorly installed venting or dipswitches not set correctly for elevation, pipe size or length.

On a unit that has been installed for some time and only recently exhibits the 90 error code, it could still be related to the above but more likely attributed to a dirty unit due to lack of maintenance.

Example of a lint filled fan causing an error code 90.

To find out whether the unit is struggling to breathe or it’s not hitting temperature, you can use the maintenance monitors which you access through the remote control. The following video demonstrates getting into maintenance monitor mode and viewing maintenance monitor (MM) 31. When troubleshooting a 90 error code, you will focus on MM31 for outlet temperature and MM49 for fan speed.

Once in MM mode with the heater running, check MM31 to see what temperature is leaving the unit then continue up to MM49.

Under normal conditions with the front cover of the unit ON, MM31 should be + or – 2 degrees from the set temperature. If the unit is set to 120, MM31 readings of 118-122 and anything between would be considered normal. If MM31 readings are considerably lower than the set temp of the unit, there is an issue getting enough gas into the burner for it to heat the water properly. This could be an external or internal problem. Generally speaking, gas related issues should be handled by a plumber or servicer trained and licensed to safely handle them.

Under normal conditions with the front cover of the unit ON, MM49 should always display 100 meaning the fan is operating at 100%. Anything higher that 100, for example 105, 107, 110 etc would indicate there is a problem pulling in the fresh air needed for combustion or expelling the exhaust from the unit. The heater responds to this by increasing the fan speed above normal to compensate but since it’s an abnormal condition, it produces a 90 error code.

Common user serviceable items that could cause an error code 90 are the areas where air is pulled into the unit whether through the front cover, the internal air filter screen, an elbow on top of the unit or the intake termination on the roof or sidewall.

Additionally the exhaust terminations should be free and clear of any debris or obstruction.

Internally, the burner, fan or heat exchanger fins could have excessive build up restricting the movement of air into or out of the unit. This will require removing parts from the unit in the gas system and should be performed by a professional.

For further technical assistance call 866-766-7489 or use our Contractor Locator to find a local service representative.

Example of heat exchanger fins packed with soot due to prolonged use in a dirty environment and lack of regular servicing.

Applicable Models

  • Models: ALL MODELS

The following video outlines some basic maintenance for a Noritz Tankless Water Heater, which may help resolve 90 error codes:

Updated on March 2, 2022

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