I'm having trouble with my Mastertemp 400 also, same problem as this one. I don't get an error code, the heater just turns off after running for 3-4 minutes and the SFS led lights up. I've replaced the thermal regulator and stack flue sensor and I'm still having the problem. The heater was working fine a few weeks ago. The heater is 7 years old and gets used on weekends in the spring and late summer mostly. The control board was replaced about 4 years ago. I feel like every year I have to replace something on it. I'm about ready to stick a grenade in the thing and blow it up.