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Diagram shows the common from transfomer as being BLUE. Locate the transformer and you will find 2 low voltage wires. One is Common and the other goes to R at thermostat.That is the common I am referring to.
Common goes directly to each component( fan relay, heat relay, compressor contactor). The the wire that goes to R at the thermostat is considerd 24v hot.When thermostat calls for FAN, the circuit is closed sending 24 volts to fan relay. When thermostat calls for cool, the circuit is closed to the compressor contactor and when thermostat calls for heat, circuit is closed going to the heat realy or gas valve.
If tech has checked it ,yes.If the board is not sending a signal to the spark igniter(I suspect that he verified that).
The board is simple to replace and can be oredered online or purchased locally. I do suggest you replace one wire at a time when replacing board to prevent any wires from getting crossed on new board and of course do it with all power OFF