I have a couple brand new carbon monoxide detectors all testing well.
Where would this supposed Carbon monoxide come from?
I have inspected heat exchanger already, no cracks.
Also think about this,
the air in the actual heat exchanger is under much lower pressure than the air outside the heat exchanger that is being compressed by the blower.
With a crack or even a hole, the air is going to be pushed INT0 the heat exchanger which would cause the flame to look different or wavy. If really bad the flame would be pushed out of the front of the furnace where the gas and air enter.