Thanks a lot, Richappy! You are so right! The heating element turned red. So I need to cancel the order for thermostat and thermistor and order the motherboard instead. Is the motherboard the only part I need to replace? What about the thermostat and thermistor? Thanks again!

To look for a bad solder joint, is there a typical location? Or it could be anywhere? Just FYI, this fridge was purchased in 2001. I guess it is old by now. BTW, I only found one type of defrost heater for my model GSS25JFMBWW. Will the newer dual heaters fit this model? Thanks again.

Thanks, Richappy. Well, I didn't find any burnt resistors or loose solder. I checked the voltage between J9 (blue) and J7 (orange), the two I used jumper on yesterday, and I got nothing. I felt a bit brave and tried to test the thermistors. I followed the tech manual, on J1, there is no wire for pin 1, I found VDC between pins 2 and 5, 3 and 5, 4 and 5. There is also 5 VDC between J1 pin 5 and J4 pin 3 (thermistor supply voltage). I also checked the resistance for the three thermistors (fresh food, freezer and evaporator) and all three readings were around 60-80 ohm. I thought the fresh food thermistor should have a lower reading because of the higher temp. Does this mean the thermistors are good? I ordered a new main board and will put it on when it comes. Is there anything I can test after I install the new board to make sure everything is ok? Or should I just wait and see if the frost comes back? Thanks!! I feel like going back to physics lab!

Just to close the loop, it's been about 2 weeks since I changed the board. Yesterday, it was confirmed that the defroster came on by itself. Therefore, the problem is solved! Richappy, thanks a lot for your help!

Pull off the power connector on the motherboard and stick a jumper between def. and line pins. If no defrost current, you have a bad defrost heater or thermostat.