Where is the flame sensor? I have a fairly modern furnace (<10yrs ?) that has a honeywell glofly ignitor. The exhaust fan starts, the glow plug glows, the gas starts and flame burns for six seconds then goes out. Sounds like a dirty flame sensor right? Problem is I cannot find a flame sensor. Can the ignitor act as a flame sensor?
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Honeywell glow fly does not work for a sensor so if someoene replaced the original ignitor with that it will not sense the flame. If you had a flame sensor it would have one wire to it and would be located at the opposite end of the burners from the ignitor.
If you have no sensor then the ignitor senses it and you will need to use the original type ignitor
Rolo
The flame sensor is usually mounted to the gas burner electric igniter bracket, if the igniter glows and the gas turns on then shuts off after flame, it may be the flame sensor. On most newer electronically controlled furnaces, there is a 1 inch clear window on the lower fan cage cover that you look through to see the fault code LED, then when you pull the lower cover there will be a control fault code LED blink chart glued to the back of the lower cover. Yes, your scenario sounds like the flame sensor :)
Add, the control board doesn't know if the flame started or the igniter is bad, it turns on the gas valve after igniter preheat cycle (usually 15 seconds) an shuts the valve off if there's no positive flame sensor input to the control after 5 seconds. The control has no way of knowing that the 120 VAC igniter has lit the gas burner.
Jackrabbit Slim
It obviously has to be in the flame. Its usually attached to the burners with a single 1/4" hex screw. At the base of the sensor, there is a ceramic collar. All furnaces have flame sensors and they are all in the path of the flame.
William
You can watch people clean flame sensors on Youtube and see where they are located. That is the easiest way to locate it.