Furnace Maintenance: Checked operation of thermostat. Changed 1(20x30x1) return air filter and also cleaned grille. Checked for gas and co leaks. Cleared pressure ports and tubing. Pulled and polished flame sensor. CHECKED HOT SURFACE IGNITER AND FOUND BURNT OFF SILICONE AND WHITING. checked amp draws on blower and inducer motor. Checked capacitor mfd ratings 10. CHECKED COMBUSTION 14.8% OXYGEN AND 12PPM. static total .48" w.c. Temp rise 58 degrees. Replaced failing hot surface igniter and combustion after balance was 12.2% oxygen and 65ppm. Could not get combusrion any higher without popping high limit switch. Installed Lennox UV germicidal light to help with alllergens, bacteria’s, viruses and mold!