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Does the unit fire up then go out or not fire at all?
Could be a bad or fouled igniter.
Do you have a multi-meter?
Ok, first check the 3 amp fuse where the electrical feed enters the inside of the unit. If that fuse is intake establish a call for hot water (run water in a tub to ensure adequate flow) then check for 120 vac across the grey wires connected to the igniter.
Service manual says you need a new board.
Ok, the bad board/ no igniter voltage applies to an 11 error which also implies the flow sensor has triggered a call for hot water. The fan should be running first when there is a call for hot water
No fan could indicate a bad flow sensor. A couple things to try. First cleanout the filter on the cold water inlet at the unit. Then try again. If still no fan check voltage to the fan and voltage into the unit. That means check the voltage at the transformer primary (120 vac) and secondary.
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