CO at 18 ppm. O2 9%. Checked refrigerant levels in straight cool AC; homeowner mentioned system leaked previously. Burners are clean. Gas valve is good with no sign of leaks. Wall filter changed by homeowner. Checked amps, caps, volts, temps, and pressures. Blower has some dust, but no blockages that would require cleaning. System lights off cleanly with a good flame. Inducer also working as intended. System is functioning normally.