If the fridge is not defrosting it can be one of three parts.
There are two that you have mentioned the other is the timer.
The timer is located in the fridge side around the controls or at the back of the fridge around the compressor.
There is a knob that sticks out the timer that you can turn using a screwdriver.
By slowly turning this you can manually put the fridge in the defrost cycle.
If there is a great amount of ice build up it will take awhile to see this start to defrost.
If you have defrosted the fridge then let it run until the fridge reaches operational temperature.
Here is a picture of the timer.
Here is a picture of the defrost heater and thermostat.  |