For flashing FL or solid FL errors, often you can tweak the pressure switch to get more life out of the switch... or at least to get the spa operating for now.
These are the most common switches and how to tweak. See How
This is the most common style pressure switch to adjust. It may have steel threads (shown) which is preferred or the more economical plaster threads.
On left is shown a metal clip that may or may not be present. If it is, to adjust remove. Typically you do not have to replace when final adjust is confirmed.
The black adjustment wheel determines how much pressure is needed to close the micro switch on top and tell the board sufficient water is flowing to allow for the heater to turn on.
If the wheel high you might get a solid FL or similar that is telling you the switch is always closed. If the wheel is low, you likely will get a flashing FL meaning when the pump is on, there isn't enough water flow to allow the switch to close.
Adjusting the wheel determines how much or how little water flow is needed for the flow over the heater element to heat the water but not overheat.
As a rule we always turned the wheel no more that 1/2 turn either clockwise or counterclockwise. The wheel should turn freely.
This is the most common low flow pressure switch. If may come with plumber's putty/tape on the threads (as shown) or you will need to add some to secure the threads to the heater body.
The principles above are applicable here.
The difference is you turn the screw slot on top of the switch to adjust response instead of a wheel.
The reason this is the most popular pressure switch for low flow pumps is it responds well to a low pressure flow system.
This isn't a common pressure switch anymore.
The above principles hold true with this switch.
The key is there is a gate you must slide up to be able to turn the adjustment wheel.
This is the lowest cost pressure switch that can be adjusted.
It requires the #30 Torx screwdriver to adjust.
If in the field and possible we always discarded this switch if the switch started becoming problematic.
Only thing to remember about flow switches is they ARE directional. Typically the housing and the switch itself with have arrows pointing the flow of water.
Failure of flow switches are created when the switch doesn't close and tell the board enough water is flowing.
There are no repairable parts to a flow switch so you simply replace with equal.
How Does a Pressure Switch Work?
Well, by pressure of course. The switch screws into piping or heater to get in most cases a POSITIVE pressure that causes a small piston to push up into a micro switch and close the switch. (on few applications the reverse is true and instead of positive it responds to negative or suction but this is not common application)
This simple operation is to tell the spa if enough water pressure is present to allow the heater to safely come on. In this case the water pressure and water movement are tied together. When the heater pump is off there is zero pressure and when on positive pressure, the switch activates/deactivates accordingly.
When operating normally when the heater pump is OFF and there is no pressure the piston is pulled back and the microswitch is open. This tells the unit all is functioning correctly.
When the unit turns on the heater pump the water pressure increases and pushes the piston up and closes the microswitch. On some pressure switches you can actually see that happening. When the microswitch is closed by the piston the unit now knows water is moving.
The switch can fail in several areas: a) The diaphragm that pushes the piston isn't responding or not correctly. b) The piston is stuck or poorly responding, c) The piston isn't being pushed far enough to close the microswitch, and d) The microswitch isn't closing/opening properly.
As shown on this page, switch typically can be tweaked BUT with limited success and dependability. Most often its best to replace the switch since the cost is so low.
Exception to this is if you first install a new switch and its not responding well or sporadically to the pump pressure then it can be adjusted.... Care must be taken because too far adjustment will cause a flow error code OR it can cause the heater to come on without enough actual water passing by the element and cause the element to overheat and fail.