Turn off power and gas to the furnace
Remove the flame sensor from the burner assembly
Inspect the sensor for dirt, corrosion, or cracks
Clean the sensor rod with fine emery cloth or steel wool
Reinstall the sensor securely
Restore power and gas
Start the furnace and observe operation
Measure flame sensor microamps with a multimeter set to DC microamps
Place the meter in series with the flame sensor wire
Check for a steady microamp reading during flame operation
Compare the reading to the manufacturer’s specified range
Replace the flame sensor if the reading is low or unstable
Check for proper grounding if the sensor tests good but the furnace still shuts off