I have a Tiger River Sumatran Spa that recently stopped heating. I checked the heater connectors and there is no voltage ogoing from the board to the heater. I suspect the problem is either with the Heater Control Board or the Thermistors. How can I tell which is the problem? I know the heater is OK because I have a backup and have tried swapping it out. I can see that the main control board heater Red LED is "on". The circulation pump and everything else seem to be OK.