I show how a HVAC tech tests a gas furnace flame sensor. The flame sensor sends a signal to the circuit board. The flame signal is a micro amp current signal. Without the flame sensor creating the proper flame current signal the control shuts off gas to burners. Therefore, I set my meter up to test micro amps. Next I place one electric meter test lead onto the flame sensors wire connection. Other meter test lead I connect to wire going back to control board. The signal travels through meter and back to control board. Therefore the flame sensor needs to be cleaned every year when heating system gets maintenance.