Chosen Solution
I have seen many issues with the microwaves failing to turn on when the door is closed. Small twist on that theme -> Our microwave ALWAYS turns ON when the door closes. If there is nothing programmed for cooking time the unit just hums away (and heats!) after shutting the door. I also tested the unit by programming a cook time (1 minute 45 seconds) at power level 2 to see what happens to a cup of water- it was hot as if the microwave operated at full power. During timed operation the microwave turntable spins and the light is on. After the timer finishes the turntable stops, the light turns off and the unit hums away and continues heating. Opening the door stops the klystron and turns on the light. Seems that the power level cannot be changed.
Hi @tumma , What is the model number of the microwave oven? The door interlock switches prevent the oven from turning on when the door is open and it shouldn’t operate until the control board starts it. So it appears that it may be a control board issue. Sticking relay contacts perhaps if the control board has relays on it of course Be safety aware when working on a microwave. The voltage stored in the HV capacitor (>6000V DC) can cause serious injury and the capacitor needs to be correctly discharged before commencing any work inside the oven once the cover is removed. This voltage can be stored for months even if the oven has not been connected to the power for that length of time
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 2 thermostats and door lock mechanism with 3 micro switches. As the blogs have mentioned, my first check was to be the oven over temperature thermostats. So with the oven UNPLUGGED, I began static testing with an Ohm meter. First I found that the temperature switches were series wired (with brown wires), making it was obvious that they must operate as normally closed switches, opening the circuit upon activation. This was confirmed by the fact that were wired as part of the power supply feed circuit as well. And with the wires removed, they tested closed ok. The top switch is mechanically a positive door closed switch wired normally open that closes when the door is firmly shut and a pin in the door depresses it. This switch has 2 brown wires connected to it that are series wired with 2 normally closed over temperature switches located at the air intake and exhaust. This brown wire circuit interrupts one side of the power supply line to the control board and magnetron. This switch tested normally. The middle micro-switch is normally depressed when the door latch is engaged, and the door opening button is not being pressed. 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 2 orange wires. This circuit remains open until the door latch is fully engaged and the door open button pops out. The electrical operation of this switch was highly intermittent. Removing the wiring from it produced the same results, so I removed the switch and opened it up. Inside, all appeared to be mechanically OK, but it still wouldn’t close 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 lower switch operates when the door latch mechanism is not fully engaged. I did not examine this switch, or it’s operation. iFixit myself…
