Jacuzzi whirlpool hot tub, flashes Fl-1 has new filters,

if you do not get a light showing on the topside panel indicating it is heating the first thing to check is the filters. Remove them even if they are new. If the problem persists then check to make sure both valves are completely open on either side of the pump so you have not restricted the waterflow.If the valves are open and the filters are removed and the heat indicator light still does NOT come on then you most likely have a bad pressure switch and should be changed.there are a couple of other things that would cause a hot tub NOT to heat but that is the starting place for you.
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if the light on the heater is not coming on when the tub is full of water and the pump is on then the heater is not working.
check the circuit breaker to make sure it is on.
the heater and the pump cannot be on the same circuit. They have to be on separate circuits.
Make sure they are on separate circuits.
Check to make sure you have the heater installed in the correct place.
there is a pressure heater that mounts anywhere after the discharge of the pump.
there is a vacuum heater that mounts in front of the pump in the suction line.
if you put on or the other in the wrong place it will never come on.

It kind of sounds like you have not purged air from near the pump impeller. Try adding water through the filter, often times this forces prime.

The bath heater you have is not designed to HEAT water - they only Maintain what you start with.
You need to start the bathtub with the hot water you want. It will only heat about 1-2 degrees an hour.

Each of the jets have rubber gaskets. The continual heat and cemicals starts to deterioate these and the tub leaks around the jets. This is not always noticeable do to the fact that the water is hot and tends to evaporate. But even a small leak multiplied by all the jets adds up. 1996 is a long time and the replacement of these gaskets are overdue. I hope this has helped it would be the first thing I would look at given the age. Hope this has helped good luck and have a great day. Jeff

A flashing “FL1” display means the flow switch is
malfunctioning open, the filter cartridge is excessively
dirty or an “air lock” condition has occurred at the
circulation pump intake.

bleed pump check fuses and connections

Check the jet pump, also where the filter is there are two 90s plastic. make sure that they are both in the water, if all those check out then check the switch for the jets. It sounds like the pump is bad and is not pumping the water. Even if the motor works and the internal parts of the motor seal gets wet then it all needs to be replaced and rebuilt. Very easy to do yourself. Like always be careful and unplug the unit.

Try turning the jets clockwise while they are running and see if this makes a difference.