Often these will trip because the rocks are packed too tightly in the heater, holding the heat inside the heater instead of letting the heat flow up and out. "Fluff" up the rock to allow more air flow through the heater. Also over time after many cycles of heating and cooling the rocks can settle in and the become too tightly packed as well.
Other parts are available as well.
To supply to correct parts for your sauna we need to know the make and model. The comon parts are: timers, thermostats, hi limit/resets, contactors and electrical terminal strips. Some of the older sauna heater parts are no longer available. Feel free to call 800 426 3929 or