This issue is common amongst microwaves. The issue is within the control board. The main control board houses relays which have different functions. You can look at your wiring schematic and determine which relay (of I think 7 or 6) is the relay for power to cooling fan. Once you’ve located this relay, you can take an x-acto knife and cut the top completely off of this relay and you’ll see that the relay has fused together. Take a small flat head screw driver and pry the contacts apart and use a very small piece of sand paper to sand away any oxidized metal and reinstall control board. Unit should now function per manufacture specifications.