Performed preventative maintenance for gas furnace. Customer mentioned the system not maintaining set temperature all the time. Inspected all the electrical components. Confirmed proper high and low voltages. Replaced the air filter due to the customer installed filter that while it was new was way too restrictive for this system. Heat was exceeding 200°F which is over the max design temperature. Once I installed our 16x25x1 air filter the system maintained standard system supply temperature of 136°. Inspected gas piping for any leaks. Tested the blower and inducer amp draw. Both were operating within manufacturer specs. Performed a combustion analysis on the furnace. System is operating at 80.4% efficiency with a CO level of 16ppm. Cleaned the burners, flame sensor and burner compartment. Flame sensor was also very dark from carbon. If problem happens again it could be flame sensor. In my opinion it was over heating due to new filter too restrictive though. System is operating properl