With nothing to loose... I would take a knife and cut down the back of the black plastic removing the flap. That will expose the LED's and Photo detector and let you slide the window out and clean it. Seal is all back up with tape...
And I would order a 4 wire board sensor cable too... It will let you test them from the top of the machine. See part number on the 2011 parts order... the last time I ordered parts...
AL