If you take off the right side equipment panel, you should be able to see the flow switch. It is a clear Tee with a wire coming off the top or bottom of it, depending on how it is installed. There are two pieces inside the flow switch that push together when the circulation pump is running. If you get a flashing FLO error, check that switch. If the two pieces are solidly touching, and you are still getting a flashing FLO error, then the flow switch is the problem.