What causes Noritz Tankless Water Heater 12?

Inlet gas pressure dropping below the minimum on the heater's rating plate, often only when the heater fires hard or when other gas appliances (furnace, oven) run at the same time. An undersized gas supply line or meter for the heater plus the other gas appliances in the house. Low propane in the storage tank, which Noritz says causes flame loss on propane installations. Build-up on the flame rod, the same cleaning issue Noritz describes for code 11. Improper venting, especially on Direct Vent (DV) units where the air intake draws in the heater's own exhaust or exhaust from another gas appliance. On Direct Vent Concentric models (NR83DVC and NR98DVC), using a vent other than Noritz Concentric Venting, or missing or damaged o-rings on the inner 3-inch exhaust pipe that let exhaust leak into the air intake. In California, a tripped earthquake safety shutoff valve on the gas line near the meter.

How do I fix Noritz Tankless Water Heater 12?

If you smell gas, stop, leave the house and call your gas utility from outside. Don't reset or relight anything. Make sure the gas valve on the line feeding the water heater is fully open. On propane, check the tank level. Noritz notes that a low tank causes flame loss. Notice when the code shows up. If it happens mostly when the furnace, oven or other gas appliances are running, or during heavy hot water use, that points to a gas supply that can't keep up, which is useful to tell your technician. Look at the outside vent terminations and make sure nothing is blocking them and that the heater's intake isn't sitting near its own exhaust or another appliance's exhaust. In California, check whether the earthquake shutoff valve near the gas meter has tripped and reset it following the procedure printed on the device, or have your gas utility or a plumber do it. Reset the heater by turning it off and back on, then try a hot water faucet. If code 12 keeps coming back, call a licensed plumber or gas technician, or Noritz technical support at 866-766-7489. Noritz says troubleshooting gas issues should be left to a professional.

When should I call a pro for Noritz Tankless Water Heater 12?

Testing inlet gas pressure three ways, as Noritz recommends: static (no gas being used), dynamic (heater firing) and max dynamic (heater firing at high demand with other gas appliances on). In all three tests the pressure must stay above the minimum on the heater's rating plate, which differs for natural gas and propane. If pressure drops below the minimum, fixing the gas supply itself (meter, regulator or line sizing) so the heater has enough gas under all conditions. Cleaning the flame rod and running through the code 11 checks for gas supply and flame sensing. Inspecting the venting for exhaust recirculation into the air intake, and on DVC models confirming Noritz Concentric Venting is used and the exhaust pipe o-rings are present and undamaged. Contacting Noritz technical support with the model and serial number if the code persists after these checks.

Code: 12🚿 Noritz Tankless Water Heater

Noritz Error Code 12: Flame Loss (Meaning, Causes, Fixes)

Q: What does Noritz Tankless Water Heater 12 mean?

Code 12 on a Noritz tankless water heater means flame loss: the burner lights, but the flame goes out or stays too weak while the heater is running. It is a close cousin of code 11, and the usual causes are not enough gas pressure, a dirty flame rod, or venting that lets the heater pull in exhaust. Check the gas valve and propane level and reset the heater; gas pressure and venting checks belong to a professional. Noritz's knowledge base says error code 12 indicates a flame loss: the unit is detecting a flame, but the flame is being extinguished during operation (a weak flame). That is the difference from code 11, where the heater can't light or can't see the flame at all. Noritz's tankless error code guide adds that with code 12 the flame doesn't stay lit, so there isn't enough heat to bring the water to the set temperature. Because tankless heaters favor temperature over volume, a weak gas supply often shows up as hot water at the right temperature but with a big drop in flow and pressure at the faucets, especially when other gas appliances such as a furnace are running at the same time.

Reviewed 2026-10-23 · Based on 2 manufacturer & service sources

What the code means

Noritz's knowledge base says error code 12 indicates a flame loss: the unit is detecting a flame, but the flame is being extinguished during operation (a weak flame). That is the difference from code 11, where the heater can't light or can't see the flame at all. Noritz's tankless error code guide adds that with code 12 the flame doesn't stay lit, so there isn't enough heat to bring the water to the set temperature. Because tankless heaters favor temperature over volume, a weak gas supply often shows up as hot water at the right temperature but with a big drop in flow and pressure at the faucets, especially when other gas appliances such as a furnace are running at the same time.

Likely causes

Inlet gas pressure dropping below the minimum on the heater's rating plate, often only when the heater fires hard or when other gas appliances (furnace, oven) run at the same time
An undersized gas supply line or meter for the heater plus the other gas appliances in the house
Low propane in the storage tank, which Noritz says causes flame loss on propane installations
Build-up on the flame rod, the same cleaning issue Noritz describes for code 11
Improper venting, especially on Direct Vent (DV) units where the air intake draws in the heater's own exhaust or exhaust from another gas appliance
On Direct Vent Concentric models (NR83DVC and NR98DVC), using a vent other than Noritz Concentric Venting, or missing or damaged o-rings on the inner 3-inch exhaust pipe that let exhaust leak into the air intake
In California, a tripped earthquake safety shutoff valve on the gas line near the meter

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.

If you smell gas, stop, leave the house and call your gas utility from outside. Don't reset or relight anything.
Make sure the gas valve on the line feeding the water heater is fully open.
On propane, check the tank level. Noritz notes that a low tank causes flame loss.
Notice when the code shows up. If it happens mostly when the furnace, oven or other gas appliances are running, or during heavy hot water use, that points to a gas supply that can't keep up, which is useful to tell your technician.
Look at the outside vent terminations and make sure nothing is blocking them and that the heater's intake isn't sitting near its own exhaust or another appliance's exhaust.
In California, check whether the earthquake shutoff valve near the gas meter has tripped and reset it following the procedure printed on the device, or have your gas utility or a plumber do it.
Reset the heater by turning it off and back on, then try a hot water faucet. If code 12 keeps coming back, call a licensed plumber or gas technician, or Noritz technical support at 866-766-7489. Noritz says troubleshooting gas issues should be left to a professional.

🛠 When to call a pro

Testing inlet gas pressure three ways, as Noritz recommends: static (no gas being used), dynamic (heater firing) and max dynamic (heater firing at high demand with other gas appliances on). In all three tests the pressure must stay above the minimum on the heater's rating plate, which differs for natural gas and propane.
If pressure drops below the minimum, fixing the gas supply itself (meter, regulator or line sizing) so the heater has enough gas under all conditions.
Cleaning the flame rod and running through the code 11 checks for gas supply and flame sensing.
Inspecting the venting for exhaust recirculation into the air intake, and on DVC models confirming Noritz Concentric Venting is used and the exhaust pipe o-rings are present and undamaged.
Contacting Noritz technical support with the model and serial number if the code persists after these checks.

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.

Noritz Error Code 12: Flame Loss (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Related codes

Sources