Also, ensure the hole for the IR light sensor on the left side is not blocked.  (See image below).  Don't clean it unless you see frosting/crumbs/icing blocking the hole.  If you need to clean it, insert a toothpick just far enough to remove the blockage (too far and you could damage or transfer frosting to the IR filter lens).  Use isopropyl alcohol or a damp cloth to clean the outside of the sensor assembly. 

Article ID 090721B