12
Noritz Tankless Water Heater
Severity: CriticalWhat it means
Noritz code 12 means the burner lit fine, ran for a while, then lost the flame.
The classic cause is gas pressure sagging under load — an undersized or long gas line, or the furnace or range kicking on and starving it.
It's also a marginal flame-sensing rod, a partial vent restriction, or wind hitting the vent terminal.
The symptom is hot water that suddenly turns cold mid-shower.
Affected Models
- Noritz NRC66, NRC83, NRC98, NRC111
- Noritz NRCP series
- Noritz EZ98, EZ111
- Noritz NR50, NR66, NR83, NR98
- Noritz GQ-C series
Common Causes
- Gas pressure drops under load — undersized or long line, or shared with the furnace/range
- Propane tank running low (the burner dies as demand peaks)
- Flame-sensing rod dirty or weak — fine cold, loses the signal as the burner area heats
- Partial vent or intake restriction choking combustion mid-run
- Strong wind across the vent terminal snuffing the flame
- Condensate backing up near the burner from a clogged drain
How to Fix It
-
Notice the pattern.
Does code 12 hit when the furnace, a gas dryer, or the range is also running? That points squarely at a gas-supply bottleneck.
Only on windy days? Look at the vent terminal.
Random, all conditions? Lean toward the flame rod or venting. -
Check propane if you're on LP.
A tank getting low supplies enough to light but not enough to hold the burner at full fire.
Check the gauge — under about 20% is suspect.
Refill and retest before opening the unit. -
Clear the vent and intake.
Outside: clear the terminal of nests, lint, leaves, snow.
Inside, breaker off: check and clean the unit's air inlet filter.
A half-blocked screen is enough to drop the burner once it's running hard. -
Clean the flame-sensing rod.
Breaker off, cover off.
Lightly clean the flame-rod tip, inspect the ceramic insulator for cracks, reseat the wire.
A weak signal that fades as the burner heats up is a textbook code 12 cause — replace the rod if the insulator is cracked. -
Have the gas pressure tested under load.
A tech puts a manometer on the inlet while the unit fires at maximum and while other gas appliances run.
If it dips below spec, the gas line has to be upsized or rerun.
That's the real fix — a part swap won't cure a starved line.
Frequently Asked Questions
What's the difference between code 11 and code 12 on a Noritz?
Code 11 is ignition failure — the burner never lit at all.
Code 12 is flame loss — it lit, ran, then went out.
Their causes overlap a lot (gas pressure, flame rod, venting), but code 12 specifically points at something that changes once the unit is running, like pressure sagging under load or wind at the vent.