This is a 25 year old built-in over-the-range appliance. Only previous problem was 1 failed door switch (of 3 total), replaced 2 years ago. First checked all 3 door switches with continuity tester, all ok. Had advice to check thermostat switches, which were all supposedly NC (normally closed), used same tester. Found one thermo that was open, ordered and installed replacement, but oven still dead. Used heat gun to warm the open thermo (while not installed) and using tester again, found that this switch is NO (normally open). At this point, tester showed lowest door switch to be not working. Bypassing this switch enabled oven to work. Diagnosis proved to be intermittent failure of this 2-year-old switch, which I then replaced, problem solved. For good measure, ordered all 3 switches to have on hand, in case they become "no longer available" before the oven expires. For any repair, the most critical step is diagnosis. An electrical problem is often identified with a simple continuity tester, basically a flashlight with 2 wire leads, which lights up when a tested device or circuit (power removed, of course) is intact. A good tool for anyone who does d.i.y. repairs!