Customer called for no heat upon inspection I restarted the system and it started without issue. Customer mentioned the system has been having issue once a season at least and would like a quote for replacement. Performed preventative maintenance instead for the time being. Replaced the non-existing air filter 16x25x1. Inspected all the electrical components. Confirmed proper high and low voltages. System had s temperature rise of 57°. Cleaned the burners, flame sensor and burner compartment. Performed a combustion analysis on the system. System is operating at 78.6% efficiency with a CO level of 43ppm. System is operating in fair condition at this time.