there are some Goodman/Amana units that have a bad seal where there primary meets the secondary. There's a service bulletin out on it that I don't have at the moment, you could have one of those units and there should be a warranty on it. Visually finding a crack is the easiest way, I don't seem to ever many goodman heat exchangers with cracks in them, I mostly just see them leaking at the secondary. They're pretty simple to replace from what I remember but I haven't done one in a few years. Try setting the manifold on the lower end of the range and watch your combustion analyzer, I bet your CO will drop.