I'm having a strange issue with my MasterTemp 400 heater. It's only 3 or 4 years old. It comes on fine and heats the spa up to what I set it to, usually 100 degrees. But after it reaches that temp and turns the gas off, it never comes back on again, so the temperature drops in the spa and all the hot chicks leave. Instead of cycling on again, the Service Heater light comes on. Even if I shut the heater and filter off for up to a half hour, it will just go back to the Service Heater light coming on. The next day I can head it up to 100 again, but then it does the same thing after it reaches 100. There are no lights on the back of the control board coming on and no codes on the front display. But the LED on the ignition module flashes 2 times continually, which indicates 'flame no call for heat'. Any ideas what might be happening here?