Pool and Spa
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First thing I would do is turn power off remove filter from spa then turn breaker back on and run without filter in it to see if Fl code goes away, if it does you need to soak the filter in a cleaning solution overnight or replace it. If the FL does not go away then you have a water flow issue with either the pump that supplies the heater to heater or a bad flow switch. Check for good flow of water if that is good find the flow switch just next to the heater usually a see thru "T" with a probe inside, remove and replace this switch.