Hi, my name is ***** ***** I have been in the home improvement and repair field for 20 years. I will be very glad to help you today. If you unplug it for 30 minutes and plug it back in, does it remove the issue?
Was it switched off or unplugged?
If you close the door firmly does the inside light turn off at all?
How about the result of closing the door firmly? Does the light go turn off even for a second?
That is very unfortunate. The control board is needing replacing. It may just be a capacitor on the board that needs replacing if you can electronics solder. Replace the main board and that will fix it.
I understand. It is very frustrating. I am sorry that it is that way. These boards fail very regularly. Would it be something that you would repair yourself or would you have a service replace it?
If you are quite handy and if there was a scale of 1-10 for difficulty it would be about a 7. It is some surgery. If you would have someone do it for you then it will very likely be near the cost of a new microwave.
You could order a new board at your local appliance parts distributor. Just have the model and serial number.