I am having a problem with my Ducane furnace model MPGA series I have the trouble light blinking four times for me.I have checked all my registers and filter and they are open and clean.I removed the duct work and it is all clear.When I power up the fan starts and then stops in about ten seconds and then I just get a humming sound.The furnace actually started up fully one time for about fifteen seconds.Thanks for any info you can give me