Product review (5 stars): Easy to change with a screw driver/nut driver
I have replaced this part every 5-7 years.  No telling how many 'fridge's have been junked because they needed this part.  Easy to change with a screw driver/nut driver, 5 mins install.  The old part can be turned with a screwdriver through it's cycles, and if compressor & fan comes on & cools, and the heating element comes on for the defrost cycle, chances are this timer is bad.  Put a mark on the timer body and the manual control knob, (the gray circle with the hole in it in the upper center of the part's picture).  Wait about an hour & see if it has rotated any.  If not, your timers bad. This is usually accessible from the outside lower back of the fridge.  I bought 2 so I'd have a spare.  With my Amazon Visa points, only cost me $8 apiece delivered.  Sure beats buying a new 'fridge!