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Understanding the purpose of the flow switch and how it works. It's protect the heater from coming on without adequate water flow and prevents the heater from "dry firing". A serviceman would be there and gone in 30 minutes after evaluating the flow switch in this manner.There are 2 messages: 1) "flashing flo" means the flow switch did not close and disables the heater. Correct adequate water flow. Check the plastic screen in the filter bay after removing the filter for leaves, plastic, etc that could restrict water flow. I will commonly find leaves or candy wrappers.2) "solid flo" indicate the switch did NOT OPEN properly when the system shut down, upon start up "solid flo" recognizes a switch that "did not open" and everything becomes disabled. Solution, replace flow switch. Best way to analyze, power down, pull the sensor loom out and put it back in, the circuit board now recognizes the switch opened and when it starts up you can see the fingers close in the transparent fitting and all functions work. Another way is to use your meter to measure resistance of the switch which should be infinity, or open. If it shows any resistance, replace it. Flashing flow is either a bad circ pump, or inadequate water flow caused by dirty filter, obstruction, or something sucked in when the filter was removed during operation.