I have a 2001 Hot Springs jetsetter spa that sometimes refuses to quit heating when I lower the temperature. I noticed that sometimes I do not hear the familiar click click of a relay or something when I turn the temp down from like 104 t0 97 to save energy costs. If I then turn the jets on, and run for awhile it will drop temp and stay at the set temp. I know it isn't a problem due to ambient temperature because we are still in the 40 degree range here in Ohio. I have replaced both the hi limit thermistor and the thermistor control sensor but no change. Could this be the relay board or something else I need?
Thanks for any help.

That's probably because it's not the heater that's keeping it hot.

A small tub like the jetsetter may maintain over 100F (sometimes much over) without the heater ever coming on.

Try putting it in summer mode. This will shut down the little 24 hour circ pump 8 hours a day. The little circ pump is the source of the extra heat. (about 65 watts that accumulates over time) If you don't use it often, it will stay surprisingly warm, even with the heater turned off.

If you have an amp clamp, I'd put it to good use to confirm the heater is indeed staying on when you've turned down the temp before replacing any other components.