Flow switches on pressure washers are used to sense when the water is moving on the high pressure side of a pressure washer. The flow switch on a pressure washer is usually used with the burner/boiler system to shut it down if there is no water running through the system. Flow switches are used in the system often as a fail save along with the unloader valve in such a way that if the gun is close and the unloader valve is in bypass mode this will stop the boiler, but if the unloader sticks the flow switch will still stop the boiler as it will detect no water running throught the pressure washer.
Problems with the burner on a pressure washer can sometimes be put down to the flow switch that has become demagnetized, the flow switch even in position will not give a reading back to the pressure washer, a new flow switch is required or a magnet or reed switch needs replacing.
Pressure Washer Flow Switch - High Pressure Equipment - Burner Boiler Accessories & Replacement Parts.