What causes Noritz Tankless Water Heater 99?

A blocked or obstructed exhaust vent (flue). A blocked or obstructed air inlet or air intake vent. Repeated abnormal combustion, where the gas and air mixture stays out of specification (the same family of problems behind code 90). A heater near the end of its service life (Noritz's EZTR40 flow chart ties 99 to the unit's life being close).

How do I fix Noritz Tankless Water Heater 99?

Look at the outside vent terminations for the exhaust and air intake and clear anything blocking them, such as snow, leaves, a nest or debris. Don't take the vent pipes apart. Turning the power off and on won't clear 99. Noritz says the lock has to be reset by a Noritz agent, so call for service instead of repeatedly cycling the power. Write down your model number from the rating label, the heater's age, and whether it showed 90 or 901 to 903 before 99. That helps the technician decide between a repair and a replacement. If the heater is older or has had repeated combustion codes, ask the technician whether it's near the end of its life. Noritz's EZTR40 chart says that when 99 means the unit's life is close, the unit is replaced. Make sure you have a working carbon monoxide alarm near the heater, and if you ever smell gas, leave the area and call your gas utility right away.

When should I call a pro for Noritz Tankless Water Heater 99?

Checking for abnormal combustion and inspecting the flue and air inlet for blockage or obstruction, per the Noritz error table. Reading Maintenance Monitor 48 (initial gas proportioning valve output correction) to see whether the unit has reached the end-of-life condition Noritz describes for 99. Checking supply and manifold gas pressure, venting setup and DIP switch settings against the service manual, the same checks Noritz gives for combustion code 90. Resetting the lock (power off, wait 10 seconds, power on, then hold Mode and Maximum Manifold Pressure Set for more than 5 seconds within 10 minutes) once the cause is fixed, or replacing the unit if it has reached end of life.

Code: 99🚿 Noritz Tankless Water Heater

Noritz Error Code 99: Combustion Abnormality Lockout (Meaning, Causes, Fixes)

Q: What does Noritz Tankless Water Heater 99 mean?

Code 99 on a Noritz tankless water heater means a combustion abnormality bad enough that the unit has locked itself out. Noritz's service manual says to check for abnormal combustion and a blocked flue or air inlet, and that a plain power reset won't clear it: a Noritz agent has to reset the lock. On the EZTR40 (NRC663-FSV), Noritz adds that 99 can mean the heater is near the end of its life, in which case the unit is usually replaced. Noritz lists error code 99 as Combustion abnormality (Unit is locked), and both the display window on the heater and the remote controller show 99. It is the step past the combustion warnings: codes 90 and 901 to 903 mean the burner's fuel and air mixture is off and the unit shuts off, while 908 and 99 mean the unit has locked itself and won't restart on its own. The service manual's checkpoints for this lockout are abnormal combustion and a blocked or obstructed flue, and the remarks column says to contact the nearest Noritz agent to reset it. For the EZTR40 (NRC663-FSV), the diagnostic flow chart says 99 happens when the air inlet or exhaust is blocked once the unit starts operating and the life of the unit is close. Noritz tracks this with Maintenance Monitor 48, the initial gas proportioning valve output correction: when the life of the unit is not close, a combustion problem gives 901 and the unit shuts off; when the life of the unit is close, it gives 99 and the unit locks. In that case Noritz's chart says basically to replace the unit. A technician resets the 908 and 99 lock by disconnecting power, waiting 10 seconds, reconnecting it, and then holding the Mode and Maximum Manifold Pressure Set buttons together for more than 5 seconds within 10 minutes. Exact behavior varies by model, so check the error table for your heater's series.

Reviewed 2026-10-24 · Based on 3 manufacturer & service sources

What the code means

Noritz lists error code 99 as Combustion abnormality (Unit is locked), and both the display window on the heater and the remote controller show 99. It is the step past the combustion warnings: codes 90 and 901 to 903 mean the burner's fuel and air mixture is off and the unit shuts off, while 908 and 99 mean the unit has locked itself and won't restart on its own. The service manual's checkpoints for this lockout are abnormal combustion and a blocked or obstructed flue, and the remarks column says to contact the nearest Noritz agent to reset it. For the EZTR40 (NRC663-FSV), the diagnostic flow chart says 99 happens when the air inlet or exhaust is blocked once the unit starts operating and the life of the unit is close. Noritz tracks this with Maintenance Monitor 48, the initial gas proportioning valve output correction: when the life of the unit is not close, a combustion problem gives 901 and the unit shuts off; when the life of the unit is close, it gives 99 and the unit locks. In that case Noritz's chart says basically to replace the unit. A technician resets the 908 and 99 lock by disconnecting power, waiting 10 seconds, reconnecting it, and then holding the Mode and Maximum Manifold Pressure Set buttons together for more than 5 seconds within 10 minutes. Exact behavior varies by model, so check the error table for your heater's series.

Likely causes

A blocked or obstructed exhaust vent (flue)
A blocked or obstructed air inlet or air intake vent
Repeated abnormal combustion, where the gas and air mixture stays out of specification (the same family of problems behind code 90)
A heater near the end of its service life (Noritz's EZTR40 flow chart ties 99 to the unit's life being close)

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.

Look at the outside vent terminations for the exhaust and air intake and clear anything blocking them, such as snow, leaves, a nest or debris. Don't take the vent pipes apart.
Turning the power off and on won't clear 99. Noritz says the lock has to be reset by a Noritz agent, so call for service instead of repeatedly cycling the power.
Write down your model number from the rating label, the heater's age, and whether it showed 90 or 901 to 903 before 99. That helps the technician decide between a repair and a replacement.
If the heater is older or has had repeated combustion codes, ask the technician whether it's near the end of its life. Noritz's EZTR40 chart says that when 99 means the unit's life is close, the unit is replaced.
Make sure you have a working carbon monoxide alarm near the heater, and if you ever smell gas, leave the area and call your gas utility right away.

🛠 When to call a pro

Checking for abnormal combustion and inspecting the flue and air inlet for blockage or obstruction, per the Noritz error table.
Reading Maintenance Monitor 48 (initial gas proportioning valve output correction) to see whether the unit has reached the end-of-life condition Noritz describes for 99.
Checking supply and manifold gas pressure, venting setup and DIP switch settings against the service manual, the same checks Noritz gives for combustion code 90.
Resetting the lock (power off, wait 10 seconds, power on, then hold Mode and Maximum Manifold Pressure Set for more than 5 seconds within 10 minutes) once the cause is fixed, or replacing the unit if it has reached end of life.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Carbon monoxide alarm

Keep a working CO alarm near any gas water heater, especially one showing combustion codes.

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⚙

Gas leak detector

Handy to have near any gas appliance.

Search eBay →

⚙

Tankless water heater flush kit

Regular descaling is part of the maintenance that helps a tankless heater reach its full service life.

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Sources

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.

Noritz Error Code 99: Combustion Abnormality Lockout (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Carbon monoxide alarm

Gas leak detector

Tankless water heater flush kit

Related codes

Sources