Noritz Code 90: Combustion Abnormality Fix

Quick Answer

Noritz Code 90 indicates the unit detected abnormal combustion conditions. The most frequent cause is exhaust gas recirculation - when the exhaust and intake vent terminations are too close together, the unit pulls in its own spent combustion gases instead of fresh air. Noritz requires a minimum 12-inch separation between exhaust and air intake terminations at the exterior wall.

I often see this code as a seasonal ghost that only appears during high winds or extreme cold. It typically signals that the unit is choking on its own exhaust gases, which can lead to heavy soot buildup inside the heat exchanger. If left unaddressed, this doesn't just stop your hot water, it can actually cause permanent heat damage to the internal burner assembly.

NoritzWaterheaterSeverity: highDifficulty: intermediate72% DIY Success

Time to Fix

30–120 min

Difficulty

intermediate

Parts Cost

$25 – $65

Tools Needed

Tape measure, Flashlight

What Does the 90 Code Mean?

While the manual lists this as a combustion fault, think of it as a breathing problem. Nine times out of ten, the issue is outside the house at the vent terminations, not a broken part inside the machine. Before you start pulling panels, we need to look at how the air is moving around those exterior pipes.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Exhaust and intake vent terminations too close together40%

Blocked exterior vent cap restricting exhaust flow24%

Failed or fouled combustion sensor22%

Gas supply pressure too low for proper combustion14%

Symptoms You May Notice

Code 90 on the display and the unit's completely locked out, won't fire at all until you manually cycle the power
The burner lights for maybe 30 to 60 seconds then cuts out mid-cycle right when the water's just starting to warm up
A faint smell of car exhaust or something burning near the air intake pipe, especially right after the unit tries to start
The code only shows up on cold windy days and then disappears when the weather calms down, making you think it fixed itself

Can you reset a Noritz waterheater to clear the 90 code?

Turn the power button off on the remote controller, wait 30 seconds, then turn it back on. If you do not have a remote, unplug the unit from the wall outlet for a full minute. Keep in mind that a reset only clears the display, you must run a hot water tap for at least five minutes to ensure the code does not trip again under load.

Tools Required for Diagnosis

Tape measureFlashlightDigital manometer (for gas pressure test)MultimeterPhillips #2 screwdriver (for access panel)Scotch-Brite pad (for sensor cleaning)

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name	OEM Number	Estimated Price
Exhaust Vent Cap (Concentric or Separate)100162001 · $25–$60	100162001	$25 – $60
Combustion Sensor100073038 · $30–$65	100073038	$30 – $65

Frequently Asked Questions

What causes Noritz Code 90?

Most of the time, your water heater's re-breathing its own exhaust. Think of it like a car running in a closed garage. If the intake pipe is too close to the exhaust pipe, the unit pulls in carbon dioxide instead of fresh oxygen. This causes a weak, yellow flame that the computer recognizes as a safety hazard, so it shuts everything down to prevent soot buildup and dangerous CO levels. It's less often a broken part and way more often a venting problem you can actually fix yourself.

How far apart do Noritz intake and exhaust need to be?

You want at least 12 inches of separation, center-to-center, between the pipes. Pro tip though: if you can manage it, put the exhaust higher than the intake. Heat rises, so placing the exhaust outlet above the intake makes it way harder for spent gases to get sucked back in. Also make sure they're not pointing directly at each other in a corner or alcove where air gets trapped. I've fixed units just by rotating one elbow 45 degrees, no parts needed at all.

Can Code 90 cause a carbon monoxide risk?

Honestly, it's a serious concern. When a Noritz hits Code 90, it's telling you the combustion process is dirty, and dirty combustion creates carbon monoxide. The unit's designed to shut down before things get lethal, but a malfunctioning sensor or someone who just keeps resetting the code without fixing the actual problem could create a genuinely dangerous situation. Always check that your indoor CO detectors are working any time this code shows up, and don't just keep clearing it hoping it goes away on its own.

Why does Noritz Code 90 happen more in winter?

Winter's the busy season for Code 90 calls, no question. Cold air is heavier and tends to linger around the terminations, while wind can swirl exhaust plumes right back around the side of the house. Plus, steam from the exhaust can freeze into icicles that eventually block the pipe opening. And if snow drifts up over a ground-level intake, that's basically an instant Code 90. If you only see this code when it's freezing outside, it's almost certainly an airflow or icing issue, not a part failure.

Will Code 90 clear itself after fixing the vent separation?

No, it won't just go away on its own. Even if the wind stops and the unit could technically run fine, the controller stays locked out for safety until you manually reset it. Cycle the power, wait 30 seconds, and fire it back up. But don't just clear it and walk away. Run a tap for five minutes and watch the display. If you don't fix the actual venting or gas pressure issue first, the code will just keep coming back, usually right when you're in the middle of a shower.

Related Noritz Waterheater Error Codes

Code: HUBNoritz Tankless Water Heater Error Codes: Complete NR and NRCP Series Guide Code: 11Noritz Code 11: No Ignition Fix for NR and NRCP Series Code: 20Noritz Code 20: Combustion Fan Motor Failure Code: RESETHow to Reset a Noritz Tankless Water Heater and Clear Error Codes

Models Known to Experience 90 Errors

This repair applies to most Noritz waterheaters with this error code. Common model numbers include:

NRCP199-DV, NRCP111-DV, NR98-DV, NR66-DV, NR711, NRC66DVN, NRC98DVN, NR981-DV

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 14, 2026