Upon inspection found system leaking carbon monoxide into the home highest reading picked up by carbon monoxide meter is at 475 PPM lowest was at 345 PPM . Found main crack thru and thru at heat exchanger tubing entrance and at stp’s on heat exchanger. Per AGA (American gas association) when a crack is found on the heat exchanger , heat exchanger or the furnace need to be replaced these are the only two options we have from this point on to properly fix the issue . System is left off at the furnace switch as well as the gas valve , also left main gas valve to furnace off ( furnace only ) at this time. Inspected evaporator coil , performed micro leak detection. Found 3 points on the serpentine leaking R22 at this time. Mega ohm compressor, compressor is ok at this time. Also performed acid test on system, acid test shows contamination on the system the system at this time ( not severe , treatable ) Also during diagnostics found an inline float switch installed on the primary drain