When I turn my spa on, it has FL1 error with no heat. My Main Pump works. My circulation pump is warm so i think it works. When I unscrew the flow switch and manually press the magnet and close the circuit, my heater turns on and FL1 goes away, and water gets hot from the pipe, but in about 15 seconds, there is the OH (overheat) error sign and the spa shuts down.....what is wrong with my spa. Please help.

likely causes in order
blocked/dirty filter - remove and run without it to see if error dissappears - buy new cartridge if over 2 years old
circ pump blockage - dismantle and remove hair and other debris which can enter pump if spa is not switched off when removing a filter for cleaning.
circ pump failure - replace circ pump
another blockage somewhere in the circulation system