Pool and Spa
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First thing to check is the fuses located on the circuit board when in doubt just replace them, next the power to the topside display is controlled by the transformer on the circuit board. You will have a plug usually in the center of the board that goes to a square looking electronics that would be it and you need to test voltage into the transformer and out of it to determine if the transformer is bad, you should have 220 volts going in and 24 volts coming out if nothing on the out then the transformer is bad and replace. Lastly try and unplug the topside from the board and look for any burn marks or damage and then plug back in and turn breaker back on and see if it lights up. Most likely it will be a fuse but check it out and see.
OK so now you need to look at all the safety switches like the temp sensor the high limit sensor and the pressure/flow switch. All of these switches need to close to allow power to go to the heater assembly. Now I would turn the spa on and turn the temp up on the heater and test for voltage across the two heater posts you need to have 220 volts ACROSS the two terminals. If you have voltage there then the heater element is bad, if no voltage look at the pressure/flow switch this is located on or near the heater and has two small wires that go up into circuit board and plug into board. Jump across these two wires to jump out the pressure switch and see if the heat starts to work if so you have a bad pressure switch.