My Whirlpool Gold refrigerator is not cooling...this has happened before & the part was replaced (under warranty) about 10 months later after purchased. Nevertheless, the refrige wasn't coolin enough to the desired 33 degrees setting. 2 1/2 years later now the refrige isn't cooling properly; the food is slowly spoiling.
While there is the possibility of the diffuser being the problem you will want to check to make sure that that this is not a defrost problem. Check and see if there is any frost build up on the back wall of the freezer. If so, you will need to test the defrost heater and the defrost thermostat. They should read closed for continuity when cold. You can test this with a multi-meter TJMA-DMT3. I will include a link at the end to assist you how to use the multi-meter if needed. If both parts are good, you will need to replace the defrost timer. Once the problem is repaired, you will need to thaw out the ice on the coils before the unit will operate properly. http://forum.appliancepartspros.com/oven-repair-including-ranges-cooktops/4810-how-check-continuity-ohmmeter.html
I am not sure if I need replace both diffuser and thermister. Is there a way to test if the diffuser is bad?
To find out of the diffuser is operational you would try to cycle unit off and back on with the diffuser off and check to see if it opens and closes.