My Master Temp 400, which is about three years old, suddenly starting reading the temperature of the water as much higher than the actual temp of the water in the spa. The temp in the spa is about 65% while the heater is saying 101 or higher, making the heater shut off. It tries to turn on again, but immediately shuts off. In looking at posts here in the forum, it appears that it could be the thermistor or the heater control board, both which appear to be easy DYI fixes. I would like to know before replacing either if I am on the right track. Any help would be appreciated.