The furnace works great. I just had it cleaned because we are thinking of selling the house. I purchased a new CO alarm a few days back so now we have 2 in the house.
"Aim small...miss small"
I second. If its producing CO then you will have to replace. You typically need a bore scope camera of some type in order to see any cracks, as the heat exchanger is not easily accessible. Thats why when guys say there is a crack without digging into the unit or seeing CO production on a meter, they are trying to sell you a furnace under false pretenses.
Stenz- I am curious. How old is your unit?