When in the tub, and running the jets, & pumps, would get an OH Overheating message then the air jets would come on to cool the system down.

Now I get a flo meassage. Filter is clean, pump seems to have lots of suction.

Pump runs for 15 seconds or so and shuts down. Heater isn't activated. Tub is slowly cooling down.

Where is the flo control sensor? Is it part of the heater? Is the heater not working, that's why no flo? How do I test the heater for fiunctionality?

A flo indicator tells the system not to activate the heater due to the fact that there no flo when there should be, You may have a problem with your pump shutting down on it's own and the system is not telling it to shut down. Check for voltage going to the pump after it shuts down to see if the voltage from the board has disappeared, if the voltage has not disappeared then you have a bad pump motor and need to replace it. If the voltage has gone away then you need to check your circuit board for voltage when activated on high speed. If the indicator light on the topside control panel indicates that the pump is on either low or high and the pump is receiving no voltage then you have a bad circuit board.