Older Beachcombers use either a flow or pressure switch which do the same thing. The motherboard can detect open and closed positions of this switch. If the switch is closed and the circuit complete with the pump not running you get a solid FL. When a tub is turned on and immediately detects a closed switch before the pump has a chance to turn on this will give a solid FL as well. This is almost always the switch. Unplug the switch it should disappear and pump should restart, if the pump restarts ok, plug it back in and the FL should stay away until the pump goes off again, then it will re-occur. Replace the switch, if the FL doesnt go away, replace the board.
(Flashing FL means the switch is open while the pump is running and wont allow it to heat.)