Furnace c+t removed and cleaned burners and reinstalled. Removed flame sensor, cleaned, replaced. Checked inducer motor, working well at this time. Inspected heat exchanger by camera, found rusting in all chambers and deep etching in two. This is an indication of where future failures of the heat exchanger will take place. Removed 90% drain pipe and cleared. Checked igniter and it was working well at this time. Checked blower motor compartment and cleaned. Checked wiring HV and LV on circuit board, discovered loose terminal and tightened. Removed electrocuted mouse from the step down transformer. Checked outlet gas pressure (3.52). checked CO2 excess air both within normal parameters. Checked for CO2 in air supply, none found. Tested high limit and gas valve safety switches, both were working. Restarted furnace though start up sequence and system was working well at this time. Reset thermostat to homeowners preferences. Job complete.