The flame sensor is positioned in the burner to "sense" the flame when the burner is operating. Most codes require as least 24" clearence access to this side when the unit is installed. So hopefully you should be able to access the burners, remove the sensor (one screw), Then clean the metal...
I'm new to this board, just found it! I use my garage for all things mechanical, repairs, refinishing, and aslo hanging out on Friday nights. Nice designs on here, I just might copy some of them. Cheers!