Noritz Tankless Water Heater Code 90 [Reasons + Solutions]

Your Noritz tankless water heater code 90 indicates that the unit is operating through abnormal combustion. It’s due to an improper mixture of air and fuel.

The most possible culprits behind the issue are: 

  • Insufficient gas supply
  • A blockage in the vent and airflow
  • Dirty fan motor and so on
noritz tankless water heater code 90

Even if, the code throws frequently, keep reading this ultimate guide to know how to troubleshoot the EC 90-related issues with your Noritz water heater. Let’s go ahead!

How To Identify Why Noritz Error Code 90 Comes Up

Noritz tankless water heater EC 90 means the status of abnormal combustion in the unit. In simple words, the unit isn’t burning properly due to issues with gas, air, and venting systems. Anyway, you need to identify whether the above-mentioned factors are responsible for poor combustion or not.

In this case, you can easily test to determine if there is a problem either with installation, gas, air, or vent.

To perform the test, go into the maintenance monitor mode by switching to the maintenance monitor mode of your unit.

When we go into maintenance mode, we’ll find out critical information about the unit. It’ll help us determine the root reason that causes the issue.

Firstly, we’ll set the temperature to 120°F and run water throughout the entire test. For better results, we should run the heater at different flow rates. We’ll run it at 1GPM (low flow rate), 2.5 GPM (mid flow rate), and the highest flow rate possible in your unit. Follow these steps to go the maintenance mode:

  • Press the Power (on/off) switch to turn the remote control off.
  • Start to press and hold both the up and the down keys simultaneously until the 03 code appears.
  • Restart the water heater so that the priority and the power lights turn on.
  • Continue to press the UP button until you see the temperature setting at the outlet on the maintenance monitor 31MM.

And then, you will see the water temperature at the outlet. Finally, now you are in maintenance monitor mode.

Write down the following number when the fixture is running all the time. Here is a list of what each valve in the maintenance mode represents in your heater.

Operational State of UnitValue/Rate Should Be
Flow Rate14
Inlet water temperature30
Outlet water temperature31
Heat exchanger temp32 (It may vary for a smaller unit as there is no bypass)
Fan Speed Correction49

For example, MM# 31 represents the outlet water temperature and MM# 14 represents the flow rate of the water. Hopefully, it’s clear to you by now. Using the readings at each of these codes, you can narrow down with these calculated numbers whether the trouble is with gas, air, or venting.

Note: When you troubleshoot the Code 90, you should focus on MM31 for outlet temperature and MM49 for fan speed.

Noritz Tankless Water Heater Code 90 [Reasons + Solutions]

Here, you will discover why you’re getting the code 90 and how to clear out it from your Noritz tankless water heater display. So, let’s get started right here… 

Note: Read our ultimate guide on how to fix Noritz tankless water heater problems & error codes.

1. Gas Supply Issues

If there are gas supply issues in your Noritz tankless water heater, the unit won’t receive enough fuel and there’ll be combustion issues. Luckily, we’ll know if there are any gas supply issues for sure by using the maintenance mode feature.

After getting access to MM mode with the heater running, the burner light will turn On and the water temperature will start to rise.

In this case, if the unit is set at 120 degrees, you will see the MM31 mode should be 2+ or 2- degrees from your set temperature. In other words, the MM31 reading range may vary from 118 to 122. However, this is considerable and a normal mode.

But if the MM#31 mode is detected lower than the set temperature of the unit, there is an issue with getting enough gas supply into the burner to hit the water accurately. This may be an external or internal issue with your heater.

Plus, when your unit starves from enough gas supply, it means:

  • The Burner isn’t getting enough gas
  • The manifold gets obstructed
  • The burner gets clogged by dirt and residuals
  • Improper inlet gas pressure


First, make sure the gas valve is fully turned on. If the gas valve isn’t fully opened, a proper amount of gas won’t flow into the burner for proper combustion.

Plus, if the burner gets blocked or clogged by debris or fuel residual, the combustion will go on poorly.

Follow the instructions to clean the burner:

  • Ensure that the unit’s gas and electric supplies are both turned off.
  • After that, remove the front cover’s mounting screws with a screwdriver and put it aside.
  • Press the top connector to separate the wiring harness from the manifold connection.
  • The seat clamp should now be released. To remove the clamp in this situation, hold the gas manifold pipe in place.
  • Unscrew the four substantial screws to release the gas line.

Then, the line will start to loosen once those screws are removed, but it won’t completely release. In order to expose the burner, push up the gas manifold pipe and remove it.

unscrewing keys to expose burner

Now, use compressed air evenly to blow away any dirt and debris from the burner chamber.

using compressed air to clean burner

And make sure you have inspected the gas manifold pipe too. If required, clean the manifold orifice with a paperclip.

After cleaning the burner as well as the gas manifold, place the parts as you disconnected through the procedures.

Finally, don’t forget to check out the inlet gas pressure for the unit. If it’s needed, you should take help from a professional to adjust the gas pressure. You can read this article on how to check gas pressure.

2. Air Or Venting Problems

When there is no problem with the gas supply and the water heater is hitting the set temperature on MM#31.

air or venting problems

Now you should move to MM#49 in the maintenance mode for vent checking. 

Follow the instructions to ensure you are in MM mode:

Push the up button repeatedly until the maintenance mode appears MM#49. This indicates Fan speed correction.

The maintenance monitor mode MM49 should be read 100 if the fan speed correction is okay. If the reading is greater than 105, 106, or 107, be sure that there is a problem either with the air or venting related.

As a result, your heater’s fan won’t be able to pull fresh air for proper combustion. Here are the main culprits that could cause this error code:

lint-filled noritz heater fan
  • The air intake filter becomes clogged.
  • The venting length is long with many elbows
  • The DIP switches for venting or high elevation are incorrectly set up
  • Venting is blocked by dirt, debris, snow, or dead birds
  • The fan is dirty from laundry lint, grease or dust from outside, etc.


Start by checking the air intake filter first as it’s the easiest to fix. If there’s too much debris on it, pull it out with a pair of pliers and clean it under a sink. Once you get clean the dirt, your heater should work normally again.

If that doesn’t work, check the venting length. If it’s too long, follow the installation maximum venting runs as recommended in the user manual. To reinstall the venting system, it will be best to contact a licensed plumber.

nortiz tankless water heater venting system

Thirdly, ensure the DIP switches for venting or higher elevations are set up accordingly. If not, see the installation manual for the correct setting and ensure the necessary DIP switch adjustments.

Fourthly, check the venting for blockage or restriction. If necessary, remove the blockage completely. Because the exhaust termination should be free from any blockage or obstruction. So, keep the internal air screen or filter clean. 

Don’t forget to clean the fan with a brush in the following image. You can use a degreaser if needed.

brushing the restricted fan

Also, if the water heater has been installed recently but exhibits the error code 90 means either the unit is affected by the above problems or lack of maintenance. 

Hence, we recommend you install your Noritz tankless water heater by a professional plumber and keep your unit under proper maintenance to avoid such an unwanted crisis.


How do I easily erase Noritz’s fault code 90?

You can easily fix Noritz EC 90 by identifying the root reasons and fixing them. Then reset your unit by unplugging the power cord, waiting for a minute, and plugging it into an electric outlet. It will erase the code.

How to purge air out of the gas tank in a tankless water heater?

Turn on your water heater and let run the hot water faucets at full speed to purge air from the pipes and heater. 

How does a venting system work for a tankless water heater?

Typically, a traditional tankless gas water heater vents through the roof or side wall. The heater uses fans to blow out the air allowing vents to terminate on the side of the house. 


Mainly, your Noritz tankless water heater code 90 throws because of insufficient gas supply to the unit, improper installation of vent pipe, size, length, or DIP switches.

Whatever the problems, fortunately, we have compiled an ultimate guide to clear code 90 with a proper explanation so that you can easily identify the core reasons and fix this issue.

Therefore, whenever you see a problematic code with your Noritz water heater, just follow our troubleshooting guide to erase the wrong code from your heater.

Yet, if you encounter any trouble removing the EC 90, let us know by your comments. Our expert will reach out to you ASAP with a probable solution.

About Phillip

Phillip is an HVAC guy & has convenient skills in the fireplace, heater, stove & other heating systems. He has been working and writing in this field for the last 5 years. He is really passionate about writing & share his expertise through his writing to help others. More about us.

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