In the picture you will see a 1/8 inch round rod in the flame, that is the flame sensor. You need a multi meter that measures micro amps which you will connect in series with the flame sensor. The sensor is usually located on left side but on some furnaces it may be on the right side.

How the flame sensor works is by a small AC electrical charge going through the sensor and it uses the flame to carry the electrical power to ground. Burner surface to flame sensor is 4:1 ratio thus causing the AC current to change to DC current. If no flame present, dirty flame sensor, bad flame characteristics similar to flame lifting off or carbonizing there will be low or no micro amp draw and the control board will shut the flames off. After 3 to 5 tries the control board will lock the furnace out for 3 hours, which can be reset by cycling power or thermostat.

All furnaces have different micro amp readings some good at .8 while others need 6 you will need to find out the right micro amp reading for your furnace. The flame sensor has a coating on it to make it last a long time and will get worn off by over cleaning, so it is best to only clean rod when it is needed. If flame sensor requires cleaning more often then once a year it should be replaced, or check for other problems like vent stack temperature, gas pressure, or primary air need to be adjusted.

How to clean flame sensor

First find the flame sensor.
Most furnaces you will find a white wire located on the left side connecting the flame sensor to the circuit board.
Watch the furnace ignite you will see spark or orange glow in front of one burner and the flame sensor should be on opposite side, but I have seen them on the next burner from the igniter.
Shut power off to the furnace while you are working, you will find a light switch high on wall or in the ceiling.
Most furnaces have 1/4 inch screw that holds the sensor in place; any multi screwdriver without the bit will be 1/4 inch. Remove the screw(s) and disconnect wire
Once removed you can clean the sensor with a scotch brite pad, if you do not have one then just use a clean rag for now. Do not use sand paper or wire brush, as it will remove the protective coating and scratch the rod causing it to fail more often. Some people like to clean every year but I advice not to, It is best to clean only when needed since the protective coating will be removed with over cleaning
Once the rod is cleaned reconnect the wire and screws making sure the rod sits in the flame. Some times the burner and any grounding surface will get dirty and need to be cleaned as well.
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