The tub started to shut off when the temp was set at 10 then over the course of a year or so slowly starting shut off at 9,8,7...etc until the tub wont come on at all. I'm sure the tub was shutting off because it was to hot, but I'm unsure if it is the control unit or high limit switch and prefer not to buy both unless I need to.

Thanks

Your high limit switch will shut down the unit from heating altogether. But your thermostat will continue it's cycling on and off... So I'd immediately suspect the thermostat/temp control. (It's pretty much a no testing required situation).

Great Thank you!!!