Thanks for replying. Some updates here. I end up with disassembled the door switch and unplug/replug all the cables on the PCB. After I put everything together, the heating worked! Also my light bulb came in and I put it on. I mounted it back to the wall.
I cooked eggs with egg cooker. Then the second day, when I use the microwave to heat up some frozen vegetables, the heating is gone again! damn! This time, I did not touch the door interlocks, just pulled out all the cables and replug them, and heating is back now. Could it be the steam sensor that somehow got triggered by the cooking steam from egg cooker? how to nailed it down?
I attached the schematic and a view of the PCB from the service manual. Appreciate any further insight.