Check the Filter
The condition of the spa water filter influences the function of the flow switch. If a flow switch is not sensing sufficient water flow, a common cause of the reduced flow is a plugged or dirty water filter. The filter is usually located near the skimmer inlet inside the spa. Open the filter housing and remove the filter. Look for leaves and other debris in the filter housing. Clean out the filter housing and replace the filter if necessary.
Electrical Supply
A common cause of a spa not working is disruption in the electrical supply. Be sure the circuit breakers are turned on and the spa is plugged into its electrical outlet. If the power is connected and you see a "Flow" error code displayed on the controls display, try disconnecting the power for 30 seconds and reconnecting; this allows the computer controls to reset and may solve the problem.
Water Levels
Over time, water evaporates from the spa and needs to be replenished. Verify that the spa has sufficient water for proper operation. Water levels should be high enough to enter the skimmer inlet. If the surface of the water is below the skimmer inlet, the pump can’t draw the water to circulate and the flow switch won’t work. Add enough water to the spa so it will spill into the skimmer.
Broken Flow Switch
The internal parts of a flow switch can break or become stuck. To test a flow switch, disconnect the power to the spa and unplug the two wires connected to the flow switch electrical tabs. Use another short piece of wire to connect the two wires you just pulled off that connect the spa to the main control panel. This action bypasses the flow switch and completes the electrical circuit back to the main control panel. Be careful that the exposed wire ends aren't touching any metal. Turn on the power to the spa and test to see how the spa works. If the spa pump and heater now work, replace the flow switch.
Adjust Flow Switch
A pressure-activated type of flow switch usually has an adjustment dial. You can tell the difference between a pressure and a paddle flow switch by the size of the switch. A pressure switch will be small and will screw into a pipe. A paddle switch is like a piece of the pipe itself. Try turning the knob to a less sensitive point of pressure detection. If the spa now works, the adjustment may have simply been a little high.
Air Lock
After draining and cleaning a spa, air pockets can remain in the piping and not allow the pump to push water, thus affecting the flow switch. Turn off the electrical supply to the spa. Slightly loosen -- but don't remove -- one of the union connecting nuts found where the pump connects to the piping. This nut is about 3 to 4 inches in diameter and is a "female" connector that turns onto the threaded ends of the pump. When water begins to flow out of the union, hand-tighten the nut and turn the electrical power back on.