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ErrBase

Noritz Code 90: Combustion Abnormality or Exhaust Recirculation

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.

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. Other causes include a blocked vent cap, low gas pressure, or a failed combustion sensor. Code 90 is more common in cold climates where wind-driven exhaust can be pushed back toward the intake.

NoritzWaterheaterSeverity: highDifficulty: intermediate72% DIY Success
Time to Fix
30–120 min
Difficulty
intermediate
Parts Cost
$25 – $65
Tools Needed
Tape measure, Flashlight
Wrong code?

Not seeing 90 on your display?

Select what your appliance is actually doing and we'll take you to the right guide.

error code on display
HUB
no ignition
11
fan motor failure
20
error code on display
RESET

What Does the 90 Code Mean?

Combustion sensor detected abnormal burning. Check for exhaust recirculation at vent terminations before testing sensors or gas pressure.

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 displayed on controller
  • Unit starts then shuts down shortly after ignition
  • Smell of combustion products near unit air intake
  • Code 90 more frequent in windy conditions

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

Press the power button off, wait 30 seconds, then press on to restart. If Code 90 is caused by wind-driven recirculation, the code may not return immediately after reset. Address vent separation before considering the problem solved.

Tools Required for Diagnosis

Tape measureFlashlightManometer (for gas pressure test)Multimeter

Diagnostic Checklist

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

1

Go to the exterior wall where the vent pipes terminate

Go to the exterior wall where the vent pipes terminate. Measure the distance between the center of the exhaust outlet and the center of the air intake inlet. Noritz requires minimum 12 inches separation.

2

Inspect both vent termination caps for ice, bird nests, insect

3

Reset the unit (power off 30 seconds, power on)

4

Check gas supply pressure using a manometer on the test port of

5

If all above checks pass

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
Exhaust Vent Cap (Concentric or Separate)100162001 · $25–$60
Combustion Sensor100073038 · $30–$65

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs
Labor Approach
Our Verdict
REPAIR
Estimated Cost: $45 (Parts) vs $1150 (New Machine)

Frequently Asked Questions

What causes Noritz Code 90?
The most common cause is exhaust recirculation - the unit's spent combustion gases are being pulled back into the air intake because the two vent terminations are too close together (less than 12 inches apart). This gives the combustion chamber oxygen-depleted air, triggering the abnormal combustion fault.
How far apart do Noritz intake and exhaust need to be?
Noritz requires a minimum of 12 inches between the exhaust outlet and the air intake inlet at the exterior wall. They should also face in different directions where possible. Check your model's installation manual for the exact clearance diagram.
Can Code 90 cause a carbon monoxide risk?
Yes. Burning oxygen-depleted air from exhaust recirculation produces higher carbon monoxide output. The safety system trips Code 90 to shut down the unit before dangerous levels build up. Never bypass or repeatedly reset Code 90 without addressing the root cause. Test your CO detectors.
Why does Noritz Code 90 happen more in winter?
Cold winter air is denser and wind is often stronger. These conditions push exhaust plumes back toward the unit, especially when ice narrows the vent caps or when the exhaust and intake terminate close together on a windy side of the building. Code 90 is notably seasonal for this reason.
Will Code 90 clear itself after fixing the vent separation?
A soft reset is required to clear Code 90 from the display. After repositioning or adding separation to the vent terminations, press the power button off, wait 30 seconds, then press on. Run the unit for several minutes under normal conditions to confirm the code does not return.

Related Noritz Waterheater Error Codes

Code: HUBNoritz Tankless Water Heater Error Codes: Complete NR and NRCP Series GuideCode: 11Noritz Code 11: No Ignition Fix for NR and NRCP SeriesCode: 20Noritz Code 20: Combustion Fan Motor FailureCode: 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

Last verified for technical accuracy on March 14, 2026