Door open alarm although the door is closed, it won't work, please advise

I'd check the door switches first. The primary and secondary ones must go into the closed position when the door is shut.

You might also have an open thermal cut out on the outside of the cavity. There is also one on the magnetron you could check, although it's les likely to cause this problem.

Avoid all high voltage components, as there's likely to be a 4000 volt charge on the high voltage capacitor. Unpleasant to touch, and it could be fatal.

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Most of the times the intermittent problem is due to the door safety microswitch. You will have to take the cover apart and check the two or three microswitches that are actuated in the close/open action of the door. They can be located very close to the locking levers on the right side of the window.
TO check these Microswitches I suggest to use a series circuit with an ordinary filament lamp and a suitable battery or Power supply.
These microswitches have three terminals, although, maybe not all 3 used, They are: a Common, a Normally Open and a Normally Closed.
Please take care not to missconnect or change the terminal positions. A good practice is to take a picture of the connections with your cellphone prior to disconnect. Good Luck.

Possible that the lock was broken and so the door opened. You can remove the front panel and check for broken plastic or nylon pieces which can be glued using instant adhesive.
Reset the lock and switch and the door must lock in correctly

If it has the "double hook" type latch on the door,see if the hooks slide up and down.They should be spring loaded,if not,the spring attachment point inside the door may have broken off(my Sharp Carousel did this).I pryed the plastic door trim loose,and reattached the spring by using one of the slots inside to hold a metal wire clip(modified with needle-nose pliers) to hook the spring onto.Your door's design may be a little different,but a similar solution will probably be apparent after you get in there.

see this causes and fix it. God bless you
Open Lever If the microwave door won't open the open lever might be broken. This part can be replaced separately.

Door Spring If the microwave door won't open the door spring may have broken. The door spring is located inside the door and keeps downward pressure on the door hook.

Door Latch If the microwave door won't open the door latch might be broken. The door latch is inside the microwave. The door hook, which is part of the door, engages with the latch to keep the door closed during operation. If the door latch or actuator breaks the door may not open.

Door Hook If the microwave door won't open the door hook might not be releasing properly. The door hook is installed inside the door and normally releases when the door is opened. The hook is made of plastic and will break over time. When this happens it will need to be replaced, it can't be repaired.

Button Spring If the microwave door won't open the button spring might be broken internally. The button spring can be replaced.
Button If the microwave door won't open the button might be broken internally. The button can be replaced.

Handle Actuator If the microwave door won't open the handle actuator might be broken. This is a plastic part which can become brittle over time. This part is connected between the door button and the door latch assembly.

check the hooks and door release button and repair or replace, when this is broken, the switch is not close the microwave will not START.
SEE the diagram
God bless you

It is one of two things, a defective door switch, or the keypad control board has failed.

One or two of the door switches has failed, replacement only option, consider price.

There probably is a bad micro switch that needs to be changed.