Samsung microwave oven turns ON when door closes

I had a similar issue with a Samsung MS19M8000AS (purchased at from HD in July 2021).

In my case, the fan ran constantly with no heating and the controls were non-functional when the door was closed.

Upon investigation, I found that the unit had two thermostats and a door lock mechanism with three micro switches.

As the blogs have mentioned, my first check was the over-temperature switches.

With the oven UNPLUGGED, I began static testing with an Ohm meter. I located two temperature switches, one on the air intake and the other on the exhaust. These switches are series wired into a brown wire safety circuit that interrupts the power supply when incomplete. With the wires removed at room temperature, each switch tested closed.

Next, I tested the upper door latch switch that indicates the oven door is mechanically closed by completing the brown wire safety circuit when a pin on the door operates it.

The 2nd switch just below the door closed switch mechanically operates when the door is closed and the door latch is completely engaged. The moment the door open button is partially depressed, and before the door is released, this switch opens a circuit on the control board via two orange wires. This circuit remains open until the door latch is fully engaged again and the door open button pops out.

With the wires removed, I tested the switch and found it to be highly intermittent, so I removed the switch and opened it up. It appeared to be OK, but it still wouldn’t operate reliably.

I placed the switch under our Lab microscope and discovered a microscopic piece of plastic embedded in the contact surface that was almost invisible to the naked eye.

The bottom switch operates when the door latch mechanism is not fully engaged. I did not examine this switch or its operation.

iFixit myself…