Brushed and vacuumed flu pipe and heat exchanger Replaced oil filter, nozzle, and strainer Checked all electrical connections and safeties Preformed combustion and smoke tests 7.2%o2 29ppm co 0 smoke When preforming combustion test noticed an increase in o2 of.5% indicating a crack starting to form in the heat exchanger. Recommend replacing furnace in the future before the crack leads to any heating issues.