Performed preventative maintenance for gas furnace. Customer called in first for the fact the system struggled to reach set temperature on the thermostat. This is due to the Merv 16 air filter being blocked up. Put a temporary standard filter in since I didn't have the proper filter on hand. Cleaned out the burners and flame sensor. Inspected all the electrical components. Tested the amp draw from the blower and inducer motors. All are operating within manufacturer spec. Performed a combustion analysis on furnace. System is operating at 83% efficiency with a CO level of 9ppm. Cleaned out the inducer port that is known for buildup debris over time. System is operating properly at this time. Will need to schedule a return visit for replacement Merv16 (20x25x5) filter.