Turn off power to the furnace at the switch and breaker
Turn off the gas supply to the furnace
Remove the furnace access panel
Locate the flame sensor
Disconnect the flame sensor wire
Remove the flame sensor mounting screw
Pull out the flame sensor
Clean the sensor rod with fine steel wool or a non-abrasive pad
Wipe the sensor with a clean dry cloth
Reinstall the flame sensor
Reconnect the wire
Replace the access panel
Turn the gas supply back on
Restore power to the furnace
Test the furnace operation