https://www.youtube.com/watch?v=Ln1ZVCLmEng
For me the issue was an obstruction on the fan.
The back vent of the oven reveals what looks like a stationary grate a few inches below the outer grate over the fan.
A kernel of popped popcorn had fallen in.
After removing a dozen of the 1/4" hex head screws around the back and the uppper metal shroud, I could rotate the upper metal shroad up and towards the front of the oven to give a little bit better access to the upper fan area. (Not much more).
I used a pair of chopsticks, fished out the kernel and then when I plugged the unit back in, it started working.
In summary, the lower grate is actually a long barrel style fan. It needs to be able to spin without obstruction or clogs.
Chopsticks work better than a fork or needle nose plyers for this job.
The fan should kick on whenever the cook top is running or still hot to prevent damage to the upper electronics.