The Heat exchanger must be CHECKED!!!!
And how, exactly, are all of you checking the 90%'s?
This outta be a hoot...
Is that what you meant?
Also, what's the issue with Trane 90% furnaces? What section of the HX is having problems? Does it include American Standard?
I do visual, camera, and a ca. It all depends on the situation. I don't pull blowers and pull apart coils unless I suspect a problem based on the previous methods.