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9 out of 10 combustion test I do I get co readings under 50ppm. I had a furnace today that had a reading around 375ppm. Everything was clean, burner was nice and blue with no apparent flame impingement, gas pressure good, drafting ok. If there is a slight crack in the heat exchanger, I know this could cause co levels to rise, but why and how. Could someone explain? Thank you.