My refrigerator does not defrost. I have replaced the defrost heater and the thermostat but it still does not defrost. I have been told there is no defrost timer on my refrigerator. My model is GSS25ufmb ww. Is this true and would the next step be the computer board. The compressor fan is working and the freezer fan is working.

You need to check the mother board. There are usually visible burn marks on it. In any case if your heater and thermo are good it can only mean you need a board.

Hello,
This is intended as a guide to defrosting a refrigerator that has the freezer on the top. Side by sides and bottom freezers may be similar but will require some different procedures to remove covers to get at the defrost coils.

If your refrigerator is not cooling but your freezer is still working then you most likely have a problem with Ice buildup. Sometimes the ice will start to form inside the refrigerator on the bottom of the freezer compartment, even though the refrigerator section is not staying cool.

1. Empty out the freezer section so that you can remove the bottom panel that covers the evaporator coils. Side by side refrigerators often have the access panel in the back of the freezer.
2. Use a hair dryer or fan to speed up the melting of the ice. Towels will be needed to soak up the water. A wet/dry shop vac and a hair dryer work the best. Melt with one and collect the water with the other.
3. Take the couple of screws that hold the defrost coil loose and remove the heater. With an Ohm meter check the resistance of the coil. If the coil is bad you will get either and open or shorted circuit. If you get a good consistent resistance then it is okay.
4. If your heater coil is good and the fan/blower is working then you probably need to replace the defrost timer or advanced defrost control board, whichever is in your unit.
Hope this help in solving the problem,Take care....