Sounds like it could be your main contol dial thermostat if the spa is old an has a mechanical thermostat an they tend to go after awhile or if its newer an has a electrical therm then is could have something to do w/the t-stat sensor which is the temp sensor this is the silver probe you should c some where in the inside of the water pertruding from the shell of the spa they start to go an can miss read temps up an down an all spas by ul law can not go above 104 thats why there is also most likly a hi limit sensor in the spa too.what it sounds like is that your sensor is so far off that when the water temp is 72 the sensor is tellin the circuit board that it is 103, an one of the many spas safty features is shutin down the heater to prevent over heating. also the heater could of went if u live in a some what warm area of the world that is what u may get as a temp just from the suns heat. check for power at the heater or read for resistants accross the two leads of the heater if you do not get a ohms reading near ten the heater is bad make sure u shut the power down always