The defrost timer prevents blockage of the fridge vents.
The refrigerator needs to stay at a consistent temperature to prevent unnecessary energy expenditure. However, if the fridge is kept cooling for too long, ice will build up on the inside. The defrost timer makes the compressor turn on and off in regular cycles to keep the ice buildup from occurring and to keep the fridge from running too hot. Ice buildup or warm temperatures in the unit may indicate a faulty timer.
Unplug the refrigerator from the wall.
Remove the front grille at the bottom of the refrigerator. The timer is usually a square or pentagon shape.
Remove the two screws holding the timer in place.
Pull out the defrost timer and disconnect the wires holding the timer in place.
Plug the wiring into the new timer and replace the timer in its slot. Replace the screws.