I've tried all of the "priming the pump" recommendations, but I still have "FLO" and no heat. Both pumps do appear to be working.

What now?

Thanks in advance for any help!

P.S. Please pardon duplicate post -- wasn't sure whether this should go here or in the "Flow error" section.

There is a small, hidden, circulation pump that is either not working, or not working properly. This pump's only job is for non-use filteration and heating. It is located before the heater assembly. I'm not positive on your sundance model, but on mine, I have 2 jets that are dedicated to that little pump, in the footwell. If you have one or two small jets that don't seem to do much, they are probably it. You might remember them spewing out hot water when the spa was heating. Check to see if you have good flow through them. If so, your pressure switch is probably defective. If not, then your circulation pump may be the problem.