Lennox MN LGC036S2BS1G SN 5606B06672 nat gas rooftop, replaced the heat exchanger less than 1 yr ago when it was found bad on a PM. Came back a week ago to do the PM again for this place, checked the heat exchanger just to make sure and it's burnt out in the same spots as before.... thoughts? The first pic is the first time I found it bad and the 2nd pic is from when I went a week ago.

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