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Hi, my name is ***** ***** I am a Master Plumber certified by Noritz for tankless repair. A "12" error code just means that it's time for a descale. This unit should be descaled annually or more so, if you have hard water. If you are on a well, it should be descaled frequently, as Nortiz units are not recommended for well water. I have seen this problem before and it sounds like you could have a blockage in the exhaust or intake vents. This is easily cleaned. Additionally, there could be an issue with the burner not firing, maybe due to soot and debris in the burner. Lastly, there could be an issue with the temperature sensor. This would require a replacement sensor be installed. Good Luck!