My furnace won't ignite this have direct ignator ,when I turn on the heat system this send signal to microprocesor and energetized to induce fan motor for 15 sec after this time furnace turn off or code flashes 3 time which means that something wrong whit the power supply.I real checked about the power and I had my 120 vols what I can do ?

If you give us the make and model of your furnace it will be easier to help. It may not be line voltage problem but low voltage problems.