My furnace will light and come on but then flame is going out MD# IS MGHA 070 ABFC- 05 my flame is turning yellow when fan comes on , there are no visible cracks in heat exchanger. I had a metal roof put on last year, could it be I need to extend roof vent or does it sound like bad heat exchanger?

If the flame is changing color when the fan comes on it's a strong indication of exhaust gasses being recirculated into the combustion area.