Fireplace sensor that is not a thermocouple or a thermopile
That's a fairly typical flame sensor. A common remedy is to polish it up a bit with steel wool. They develop a carbon coating that inhibits sensitivity. Yours looks a bit grungy, and judging by your symptoms I'd guess that cleaning or replacing it will resolve the issue.


  A flame sensor "senses" a weak DC signal from the AC power sent to the ignitor which via the phenomenon of flame rectification in which the polarity of power sent through a flame is rectified to DC, flame is a poor conductor so the signal is mere micro amps .3 to .5 usually and the flame sensor is nothing more than a rod insulated [b]y a ceramic base so it is not in contact with anything....


https://www.quora.com/How-does-a-flame-sensor-work