FYI. Flame sensor Is NOT the same as a thermocouple You gave very little info. With that little info I took my best guess
Eta.,,,
I'm not trying to be a jerk. I literally meant to google "flame sensor". The top hit is a YouTube video on how to check and clean a modern flame sensor.
Another "guess" would be a high limit sensor if the blower comes off and on
this...and steel wool...8)