Stop Opening Your Microwave Door in the Middle of Cooking

I had an expensive over-the-range microwave that worked great for a couple years. Then suddenly, the instant you opened the door, all power to the microwave would be lost. I’d have to go reset the circuit breaker to get the microwave to power on again.

After reset, everything would look normal, but the second you opened the microwave door, bam, circuit trips again.

Given these symptoms, what could the problem be? I researched and found this video by Steve (GuruBrew on YouTube) – which notes in a linked microwave repair guide:

Problem: Fuse blows [ / circuit breaks ] when closing or opening door. Possible causes:

  • a. Defective door interlock switch(s).
  • b. Interlock switch knocked out of position.
  • c. Misaligned door.

I quickly ruled out c – the door was solid and aligned properly. That left a or b – one of the interlock switches – a $6 part.

The three door interlock switches inside most microwaves
Image credit: wallyk on diy.stackexchange

So what causes failed door interlock switches? The same microwave repair guide notes (and diy.stackexchange corroborates):

“Failed door interlocks account for the majority of microwave oven problems – perhaps as high as 75 percent. This is not surprising considering that two of the three switches [in some ovens] carry the full oven current – any deterioration of the contacts results in increased resistance leading to their heating and further deterioration. And, opening the door to interrupt a cook cycle results in arcing at the contacts.

Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Microwave Ovens

This last point is critical. I learned that I should never open the door to interrupt a cook cycle – this results in wear on the interlock switch and very likely caused the original problem. I used to do this all the time and learned my lesson!

So did I try to repair the microwave myself? No, I sold it, because at the time, I didn’t want to open up the microwave. It can be dangerous to work in there (even if unplugged) if you’re not experienced working with appliances and shorting capacitors. (Looking back, now that I have a couple of electronics projects under my belt, I probably would have attempted this myself.) Ended up buying a replacement microwave pretty similar to this $426 one from Sharp.

Ever wonder who actually makes microwaves nowadays? There are only a few companies left: Sharp, Panasonic, and LG. Other brands are mostly just rebranded with different faceplates. And you only get a one year warranty on many (all?) of them, so it doesn’t matter much which one you get.