Pool and Spa
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It says Flow and cool so that means it thinks there is no water flowing through the heater this can be a bad pump, or bad pressure/flow switch. You need to determine if there is water flow through the heater housing. Look at the heater and follow the plumbing back to whatever pump feeds the water to it and be sure that this is running and has water movement, if this is good then the problem lies in the pressure/flow switch which is located on or next to the heater housing there will be usually 2 small wires going from the switch to the circuit board remove and replace that switch and you should be good.