Brushed and vacuumed soot and debris from heat exchanger and flue pipe. Checked electrical connections and safeties. Replaced the nozzle, filter, and strainer. Performed a combustion test on unit, unit is running at 7.4% O2, 41ppm CO, with zero smoke. During maintenance noticed corrosion on the inside of the heat exchanger, indicating a slow leak between the sections. Might need to replace boiler in the few years.