I have an amana furnace about 20 years old. Model #ghi115a35. When I put the thermostate at 70 or 75, does not matter, the inducer motor comes on, the glow light is on, the gas valve will open, but will not ignite the burners. Then the gas will close, I try again, about two times, then the gas valve will open and it will ignite. After 20 seconds it will shut off. The blower motor will continue to be on and not shut off. The fan and limit is about five inches long. Again, will try to ignite the burners and they will ignite the third time and again will go off. Do you think the problem is the fan and limit? Or the orafices in the burners are dirty or clogged?

My first thought is that it is a dirty flame sensor. You need to take it out and clean it with a piece of sandpaper and then try it again.