The flow switch shuts the heater off when there is an interruption of water flow or the necessary amount of water flow is too low. This protects the heater. Check for debris in your nozzle. Also, always use the correct nozzle size with your model. Excess air in the water system can cause the flow switch to flutter. Make sure you purge the system of all air before using the machine. This will lengthen the life of your flow switch. Using the trigger gun activates this switch. Excessive “ON” “OFF” action will also shorten the life of your flow switch.