Recently picked up a non functional SM150 (2003). Opened the back and cleaned as many electrical contacts as I could find and bingo…working. I was wondering the same thing about the door. Although I do know the heating element will shut off and not work when the door is opened. It will resume once the door is closed. No indicator on the control panel as you mentioned.
The upper left portion of the display is out but the remainder of the screen is working enough where I can make out what it’s telling me. May eventually replace with the new IQ5 controller. Other that it is in pristine condition for its age.