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The other part which causes FLO error code is the Pressure/Flow Switch located on or next to heater housing. It will have two small wires going to circuit board. You can try jumping across these wires to simulate the switch closing, if this works the switch may be defective but you have to verify that there is GOOD water flow through heater, if circulation pump that feeds water to heater is not working then the switch is doing it's job and you need to fix pump.
it sounds to me like the pump housing may be clogged. I would remove the pump, take off the face plate of the pump and inspect and clean out the veins of the impeller. You may have enough water flow for low speed but not ffor high speed. After that I would consider the flow switch to be a problem.
OK so the front of every pump has a face plate usually the face plate has 10 or 12 screws around the front, once you remove the screws take a flat head screw driver and pry the face plate open once inside you will see the impeller and clean that out.