There no second chance with CO it will either make you and your loved ones feel ill or kill you is it worth the risk .
True, there is no second chance. However, not one (of 3) of my CO detectors are triggering, that is, always showing 0% ppm of CO.
Additionally, think of this: How can I possibly turn three (3) burners on my natural gas range top appliance and the three CO detectors maintain a 0% ppm? Remember, of course, that this is an open flame, burning blue steady flame (as it should). The exhaust fan above the range is not near sufficient (by way of CFM) to exhaust adequately. Yet, CO detectors are fine! How then, could a small crack in a heat exchanger produce toxic levels to a fatal level? It makes no sense.
Too, with adequate oxygen intake from outside, one (1) oxygen molecule will bond with the CO particles, forming CO2, a non-toxic noxious gas. It makes us sleepy and tired, but its generally deemed as fatal. After all, that's the principle of parking garages with direct fire MUA and exhaust fans. The two (2) units must be interlocked so as to provide sufficient replacement of the air. CO levels won't build too high with the proper introduction of outside air.
Notwithstanding the initial heating of the heat exchanger (prior to fan start), there is higher air pressure outside the exchanger when the fan starts. When the fan starts, the positive static pressure outside exchanger prevents combustion gases from migrating into the supply air.
A lot of evidence, a small crack is nothing to worry about. Doesn't mean you ignore it...but, you need not panic. Get your funds together, arrange a suitable time with the installer, then change it.