1999 Sundance Montego 750 spa.

I sucessfully replaced the circuit board last week, but unfortunately I am experiencing the same problematic symptoms that led me to replace the circuit board in the first place.

Specifically, at low water temperature, my tub heats properly and all the
jets and pumps work fine. However, if the tub is at temperature and
otherwise idle (no pumps), I get a FLO indicator on the display, locking out any topside controls, and the tub continues to heat the water to excessively high temperatures.

Resetting the tub and immediately turning on a pump will prevent this from happening and allow the tub to cool down again, but as it sits idle, after the pump(s) shut off and reaches the set temperature, the problem reoccurs.

Any thoughts on what part (outside of the controller board of course) would be causing such behavior? Somone suggested the filter well temp. sensor, but I don't get why the FLO indicator?

I would appreciate any diagnotics ideas anyone may have.

what bothers me is with the flo indicator, why is your heater still heating and not going into overheat mode?

The temp sensor is in the filter well, and if that is out of range, it will cause the display to read wrong as well as heat to the wrong temperature. Does the digital display the correct temperature, or is it lower than actual?

The kicker in this situation is the freeze/high temp sensor, located in the heater housing. That should be sensing an over heat (due to the lack of flow to give you the flo error) since no water moving will quickly overheat that heater and thus the sensor. That aparently isn't happening. Your circulation pump must be working if that isn't happening. That would mean the flow switch is defective. But your switch must be ok, otherwise it would never heat. Which leaves....circuit board.

I'm thinking that you may have gotten a bum of a replacement board.