Tech replaced inducer assembly 12/9/2019 and now not heating again. Check and find inducer assembly tube bend causing pressure restriction. Repair tube and system operating but flame sensor also bad reading .4/1.5 rated. Replace sensor for the upstairs system. System operating at this time. Find downstairs system operating but CO level reading over 3000 ppm and detector lockout on safety. Find furnace in exchanger badly rusted and system not burning properly causing dangerous co level. Shut system off. Run system for about 20 min and the house already reading 40ppm in house. Not safe to operate. Due to bad heat exchanger, we need to replace the system.