KitchenAid WRF989SDAM00 evaporator ice build-up

[sonoman]

KitchenAid WRF989SDAM00 has recurring problem with ice build-up on the evaporator coils, resulting in fan noise as the fan hits the ice.  I want it to stop. I have melted the ice but it comes back...it used to take months between melting the ice and now it takes weeks.  I want a permanent fix.

As I understand it, there are 3-4 parts that could contribute to this, the defrost timer, defrost thermostat, defrost heater, or the control board.  Sadly, several of these parts are out of stock at RC, so I may have trouble sourcing them.  As with other older appliances (this one is only maybe 6 years old...), the parts get end-of-lifed and difficult to find. 

How do I test the parts to diagnose which to purchase?  I can put my ohmmeter across the heater...but I need to locate the heater first.  Is it clear where to connect the meter and can I do it in the freezer, or do I need to fully disassemble it?

Is the thermostat the temp sensor (attached to 1"x 4" (appx) plastic with curved outside) that clips to outside the sheet metal separating the evaporator assembly from the freezer?  Where is the timer located and how do I test the timer?  And how do I verify it is not the control board...a very expensive part?

 

 thanks