Temp thermistors read temps and send the signal back to the control board via resistance. If you can find out from the manufacturer how many ohms at a certain temp you would be able read this with a volt/ohm meter and determine if it is out of whack.
Otherwise take the chance and replace the thermistor.

Hi pointerx,

I saw your posting and am curious to hear if you were able to solve your problem with the Sundance spa. Which one do you have, and have you had other problems with it? We are about to buy a hot tub, and one of the ones we are seriously considering is a Sundance model. I'd love to touch base w/ someone who has had one for a while to find out how they like it! They will include an ozone thing if we get the order in by the end of the month (which is tomorrow!??) Thanks in advance....

I have a 99 Altimar that had similar symptoms. I read online that Sundance spas from that era are notorious for having temp sensor issues, so I replaced it and have had no further problems