As long as it says flo the heater will not come on. So you replaced the flo switch and the circ pump is running? If so put your hand on the circ pump side where the filter screws in, side with screen on it and confirm you have suction. If not its probably a bad circ pump. It's possibly a bad connection on flow sensor to board, or a bad circuit board.. Filter is clean and no air lock. You need to get that error off there before replacing the heater..