96 E-250 Heat Problems

mY 96 E-250 HEAT WORKS GREAT WHEN IT BLOWS HEAT. IT WILL BE WORKING FINE BLOWING HEAT, THEN IT GOES COLD. IF I PLAY WITH THE TEMPERATURE SWITCH ON DASH THE HEAT USUALLY WOULD COME BACK. NOW IT DOES NOT IT ONLY BLOWS COLD. I TOOK THE DASH APART HOPING THAT THE MANUAL SWITCH WAS UNHOOKED BUT IT IS ELECTRICAL. I PULLED OUT THE CIRCUIT BOARD AND TESTED IT BUT IT SEEMS FINE. ANY HELP WOULD BE GREAT. I DON'T KNOW IF THE BOARD IS BAD OR IF THE FLAPPER MOTOR IS BAD OR HOW TO FIND IT AND CHECK IT. THANK YOU

this year commonly broke the blend door at the point where the eletrical motor goes in