Performed preventative maintenance for standard gas furnace. Upon arrival customer stated that he went to turn it on and he heard the inducer come on but no flame. As I started my diagnosis I found the pressure switch sticking causing the unit not to light. Replaced the pressure switch with a compatible.30 WC pressure switch I had available on the truck. Tested the system upon completion. System is operating properly at this time. Confirmed proper high and low voltages. Cleaned the burners, flame sensor and burner compartment. Customer replaces the 16x25x1 filter. Performed a combustion analysis on the furnace. System is operating at 78.9% efficiency with a CO level of 29ppm. System is fairly old and would recommend replacing in the near future but is operating as it should at this time.