I have had my Takagi T-K2 for 3 years and love it. But lately there has been a problem with it failing, giving error code 11. The machine fires about 10% of the time, but after it fires once, it fires fine for most of the day. But at night and in the morning, it always fails.
I have talked to Takagi support twice and had my gas company come out to check the pressure gas pressure (which was fine - 9 inch WC).
Here is what I know. When hot water is turned on: the fan turns on OK; the igniter sparks OK (best to check this in the dark); and it sounds like the gas switch turns on OK (ie, it "clunks").
Despite all of this, the flame does not light!! The machine repeats three rounds of igniter sparking, then gives up and reports error 11. If you try this 5-10 times, eventually it will light, work just fine, and continue working for the rest of the day.
When thinking about the issue, the obvious things can be ruled out. The gas pressure is fine; the gas line is sufficiently large (3/4" from the meter 10' away); the burners look perfectly clean, with no visible dust or grime when looking through the opened ignition plate hole. I also have been vigilent about descaling the heat exchanger and cleaning the water filter once a year.
I will continue to discuss this with Takagi customer service, but any constructive input from this group would be helpful.