The most common problem when this fan motor is not running is due to a fault in the relay board. The problem is: The relay board is difficult to get to. It is inside the rear right side of the microwave. I have replaced these with the microwave still up, but it is difficult to do. The easiest method for a novice is going to be to pull the microwave down, remove the outside cover, then attempt the replacement. To test the fan motor during a cooking mode, you can test
Please see my attached WORD document. This has some information showing you the locations of the relay board and the cooling fan. Also, I included a ladder schematic with directions.