, this vent and microwave combination would not heat when the start button was pressed, but instead the timer would count down just like it was operating. There was no noise like the normal vibration or fan sounds when the start button was pressed. It simply acted like it was going to operate, but it never actually turned on any of the components.
Microwave ovens use a series of micro switches to operate as interlocks to ensure the unit will only operate once the door is securely closed. There will be at least three of these switches aligned so they close in a specific sequence when the oven door is closed. This closure process is designed to prevent someone from just sticking something in to actuate the switch and run the microwave with the door wide open and becoming exposed to the microwave energy. Two of the switches are in place to provide power to the units high voltage section and to tell the control board when the door has been closed. These are usually normally open switches. The third switch is called the monitor and acts as a safety switch blowing the internal appliance fuse in the event any tampering is going on, and this one is usually a normally closed switch. If you find the fuse inside your microwave is now an open circuit, check the monitor switch as it may be the cause.
As I mentioned above, the control board takes it's input from one of the switches to know when the door is closed, but it is the other switch that actually provides the voltage path to the high voltage section of the microwave. So if the control thinks the door is closed because one switch is closed, while the other is still open, the result is the display will tick away the seconds, but the unit doesn't cook the food. Basically, all three switches need to be actuated by the door in the correct order for the unit to operate properly.
Because of the symptom described by the customer, I immediately removed the grill to get access to the screw securing the control panel to the chassis. Behind the panel is the bracket that holds the three interlock switches. As I said, each switch has a purpose and they are not always in the same place on all microwaves, but it isn't important because all you need to do is verify the operation of these microwave parts with a meter (the post Testing Appliance Micro Switches gives complete details). With the microwave unplugged from the outlet, I took my meter and verified each switch was mechanically operating and that electrical continuity would change as it was actuated. In doing so, I found the bottom switch was reading as an open circuit when open or closed, which is exactly what I had expected. I next removed the old switch and replaced it with a switch being careful of the mounting tabs to prevent accidental breakage. Everything was reinstalled before the unit was plugged back in. With power reapplied I tried heating some water, and it not only counted down the time, it heated the water as well.
Now if you find all the switches check out correctly on your microwave, you will then need to verify the door strikes are properly closing the switch push buttons. If you find a problem here, the interlock mount may be broken allowing the switches to move when the door is closed. Check in microwave parts for your particular model. It is also possible the mount is out of adjustment and needs to be realigned with the door strikes. There are usually a couple screws holding the mount to the chassis that allow for some minor adjusting. Or you might find the mount needs to be bent a little more toward the door. Often times, the metal housing that holds the interlock mount in place will flex due to the door being slammed causing it to eventually bend enough preventing good switch contact. Sometimes just a little adjustment can make all the difference.