I've got an older built-in tub on my deck. I started working on getting it set up this last weekend. My remaining hurdle is that the heater isn't turning off when the pump turns off. I can turn the heater off with the temp control on the hot tub or on the heater itself, so I don't believe it's the contactor in the heater--it moves freely, too. I've been paying attention, so I haven't let the heater run and trip the high-limit switch.
Is this likely to be the controller? I've been tossing parts at the tub. Conveniently, I've got an indoor tub that has the same exact equipment. I can swap out parts from the controller, which I suppose will be my next step. So far I've had to replace the heater's circuit breaker and swap out the timers. If this wasn't a very busy time in life, I think I'd actually enjoy the trouble-shooting and repairs.
PS, I've been very thankful for the advice I've received here, both directly and indirectly. Thank you to all of the contributors who make this site such a great resource!