Did a preventative maintenance on a Lennox gas furnace. During the maintenance I clean the flame sensor I also cleaned out the burner box in the blower motor housing. Also did an amp check on the inducer motor and the blower motor know everything is in range of the furnace specs. Also did a CO test which was seven and an O2 test which was 19. Also shop vac the entire unit out due to it being dirty and dusty. Also swapped out thermostat batteries. System is operating properly at this point. M#G50UH-48C-110-10 s#590El93