My father has a Seven Seas Tazmania spa and is having an issue where the water heats to 80 degree, as indicated on the control panel, but no more than that. The spa doesn't trip the GFCI and appears to work in every other sense. No one around here will service the spa.

How do I go about troubleshooting this for him? My thought is it could be one of 3 things...Control Panel, Heater element/Heater assembly or thermostat. Obviously, I don't want to buy all three parts. How do I figure out which, if any, is the offending element?

Thanks.