Flow Switch vs Pressure Switch: Most of Jacuzzi's spas used a pressure switch. If you have a 24/7 circulation pump you likely have a flow switch. If the spa has only ONE pump, the low it does the heating. If the spa has TWO pumps, the #1 pump drives the heater and pressure switch.  Whether you have a pressure or flow switch, the switches are simply on/off switches. They tell the board if enough water is moving pass the heater element. If not, the unit will not turn the heater on and give a flow error. What the board looks for is for the switch to be OPEN when the pump is off. Then it turns the pump on and now looks at the switch to be closed. If so, the board turns the heater on. IF either situation is not true, the board will not turn on the heater and will give an error message.

Flow Switch: Its fairly simple to replace. It IS directional. So make sure the arrows point in the direction of water flow. On most the housing is clear so you can see the switch. It consists of 2 parts; the flap and the post. The post remains stationary and contains the switch. The flap is AWAY from the post when the pump is off and against the post when the pump is on. A clear housing allows you to see if that is true and if not, gives indication to the problem.

Pressure Switch: Most switches are adjustable. Problem is there is a "sweet spot" that the switch must reside in. If the adjustment is pass this sweet spot, you will get errors. That sweet spot means when the pump is OFF, the switch is open. When the pump is ON, the switch is closed. Generally speaking, you can adjust "on the fly". To do this, turn the adjust wheel fully clockwise. Turn on the spa. There should be a delay and they the Low Speed or #1 pump comes on. Turn the adjust wheel counterclockwise while the pump is on. When you enter the "sweet spot" the heater relay will CLICK and the heater light will come on solid. Turn 1/3 more and stop. You are in the sweet spot.  PLEASE NOTE to be careful because you are working in an area with electronic parts exposed. If you do not feel comfortable around live electrical components, do not attempt.

See Here For Pressure Switch Adjustments