Open up the freezer section, and go down to the bottom, and look at the back wall, and see if you have a frost build up. If so behind the control panel in the refrigerator is the timer. If you turn it clockwise till it shuts, off see if the heaters come on. If you have an ohm meter, just unplug the refrigerator, and ohm out the heaters. There is also a defrost thermostat, that sits on top of the coil, but it has to be cold to check. Tom