There are 3 parts involved with defrost the timer, the defrost thermostat and the defrost heater.
To troubleshoot you force a defrost cycle.
If the heater comes on then you replace the defrost timer.
If the heater does not come on then you have to check the heater (usually around 20 ohms) and the defrost thermostat (should be 0 ohms) with a meter. The thermostat must be below freezing as it opens just above freezing.
If both measure OK then it is probably the timer.
There is also a possibility that it is a wiring problem but this is very rare.
Below is a wiring diagram it is for a MRS20WRG2 which is the cosest diagram I could find to your unit. Yours will be close if not an exact match.
http://manuals.frigidaire.com/p...f/Anderson/218905000.pdfIf 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.
4. When you start always short the meter leads together. This will tell you that the meter is working and if there is any 0 offset.
There is a good STICKY at the start of this forum about it's use.