Preventative maintenence inspection on four furnace systems. Replaced all air filters. Clean flame sensors and checked ignitors. Visual inspection of burners. Flush condensation traps on high efficiency furnaces. Run test all in heating mode from respective thermostats. All systems are operating properly and within specs for temperature rise, and blower and inducer motor amp draw. All exhaust vents are in good condition and no sign of leaks. 0PPM CO detected during testing. Systems are operating properly, efficiently and ready for Winter.