Brushed and vacuumed flue pipe and heat exchanger Replaced oil filter, nozzle, and strainer Checked all electrical connections and safeties Performed combustion and smoke tests 5.7%o2 80ppm co 0 smoke While performing the maintenance we found that the combustion chamber has fallen apart and a section is missing from the bottom. Showed the customer and informed them it may impact the performance of the furnace. While performing the combustion test we found that the o2 increased from 4.3%o2 up to 5.2% when the blower turned on indicating there is a crack in the heat exchanger. Recommend replacing the furnace before any issues arise.