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Before assuming it is a defrost problem make sure that the evaporator fan is running.
This is required to push the cold air from the freezer into the fresh food section.
Also check that the damper (louvers) between the freezer and fresh food are opening.

If both the above are OK then remove the evaporator covers and check the coils, if they are heavily iced/frosted over then you have a defrost problem.

Here is the tech sheet
http://www.servicematters.com/d...0Sheet%20-%202303922.pdf

Try forcing a defrost cycle.
If the defrost heater comes on then the board is probably shot.
Be careful not to burn your fingers!!!

If no heat then check the defrost heater and the defrost thermostat with a meter.
Note that the defrost thermostat must be frozen to give 0 ohms, it opens just above freezing.
If both OK then you could check if they are receiving power as it may still be a board problem.

Here are a couple good sites with troubleshooting help.
http://www.applianceaid.com/refrigerator.html
Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com

If you do not own a meter, I would suggest you purchase a one. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.

There is a good STICKY at the start of this forum about it's use.