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Please do the following
Turn your unit off.
Wait one minute.
Turn the unit back on.
Watch the operation.
Normally you would see the following:
Please watch your unit and tell me where your unit deviates from the description I have given.
What i have seen on INTERTHERM furnaces that are of the age you have is at the top of the heat exchanger ( your heat exchanger is made like a barrell ) it will get cracks - sometimes quite large and when the blower runs air is getting down into the burner compartment and the burner is on but not heating the exchanger. Luckily on mobile home furnaces this is a sealed combustion furnace and so you are not getting carbon monoxide in to your home but 40 years is way longer than these heat exchanger were meant to last. To check this you need to take the outer pipe off the flue and the top of the furnace off and look down on to the top of the heat exchanger and feel around for cracks . I've seen some that I could stick my fingers in. IF that is the case - no fix- replace the furnace.