This morning found the circulating pumps running but no heat or noise (fan noise) produced by the tank-less water heater. I shut off all the valves, cleaned the fine mesh screen, etc.. then pressurized the system & turned the call box to circulate water on to flush out air. Once the system was circulating and pressurized turn the T-KJr power back on. System didn't start up. Normally this would have done the trick

Pulled the panel and noticed the GFCI red LED on. Pushed reset to no avail. Turned off power. Disconnected Power to circuit board. Turn power on. GFCI LED off. Tested GFCI, works fine. Reconnected circuit board power. Turned power on, GFCI trips again. Methodically disconnected each cable plugged into circuit board. When disconnected FAN motor GFCI didn't trip.

Noticed Fan casing leaking water. Removed fan from casing. Soaked it up. Without putting it back in reconnected circuit board and turned on power. GFCI doesn't trip anymore with fan plugged in. Installed fan and circuit board. System started up and ran fine. Put the housing cover back on and turned it on. Working for maybe ten minutes then it stops working. Fan does not turn on.

Now the circuit board LED blinks four times indicating Fan or Fan pressure sensor. The fan pressure sensor is "normally open", it is open. So this wouldn't disable the fan when heat is called. Took the fan out and it worked when held in my hand. Reinstalled fan. It didn't run at all. Rapped on fan, it turned on and now stays on whenever power is applied to the unit.

question one: What can cause the fan to turn on when there is no heat call. I tried disconnecting the water flow sensor (heat call) and fan still runs as soon as power is applied. Not sure this is a analog circuit or switch circuit so that may not indicate anything. As soon as power is applied the fan starts. This may indicate something to a trained Takagi repair tech who understand the circuit board well.

Not knowing the motor circuit electronics it is hard to determine a bad motor versus a circuit board malfunction. It doesn't look like a sensor issue given the limited sensor involved in initial startup.

One more point The flame never ignites and there is no attempt to ignite. So this may indicate heat is not being called. Also even when I turn on the circulating pumps the system does not ignite flame.

4 blinks - is it the fan or the control?

question two: where did the water in the fan casing come from? It is now dry. So not an obvious leak. Condensation? from circulation cold water for who knows how long and damp air at the same time? I think so. Did this cause the fan motor issue? Did the fan motor some how effect the circuit board?