When the door is shut the microwave, in an apparently random manner, turns itself on for at least 2 1/2 minutes at a time. Right now we have the door ajar but iwe are afraid it will turn on when someone walking by shuts it.

I had one that started randomly when there was a power failure.
Never found out why. But when it did only the convection part came on. The mwave did not. U might experiment with some water in there to see if it heats up when this happens. If the mwave does come on then u should get it fixed or junk it. With the cost analysis, junk it is probably good.

It could be a loose connection that shakes when the door is closed. It could be a defect in the electronics.

Mark sounds like it is tripping a internal limit relay, weak power supply or door switch possibly. Below are links to help diagnose.
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Kenmore Microwave Model 401 80084700 40180084700 401 80084700 Parts

Kenmore Microwave Shuts Off After Few Seconds

Microwave Oven Shuts Off After 2 Or 3 Seconds How To Fix

If our Panasonic microwave shuts off after 2 seconds is it likely door...

The microwave is not supposed to shut off if you have food in it. it is designed to stay running so you will not forget your popcorn.

mine turned exhaust fan on automatically... i couldnt shut it off... had to unplug it.... no buttons worked.

HELLO.

Solution to your microwave problem is given as below.

* 1 Set your Daewoo microwave on a countertop near an electrical outlet and plug it in. Open the door and place the circular roller guide in the center of the oven cavity, then place the glass cooking tray over it.

* 2 Set your food on a microwave-safe dish and then put the dish on the cooking tray and close the door. Make sure that it latches properly.

* 3 Turn the upper knob to select a power level. Power levels include "Defrost," "Med-High," or "High." If your Daewoo has a touch pad, touch the "Microwave" button once to set the highest power level. Touch it consecutive times to lower the power level.

* 4 Turn the lower knob to set the cooking time. For cooking times less than two minutes, turn the knob up past the two-minute mark, then back to the correct time. For touch pad models, enter in the cooking time with the numerical keys and then press "Start."

* 5 Open the door at any time to stop the cooking and check the temperature of your food. Shut the door to automatically continue cooking, or turn the timer knob to the right to add more time. For touch pads, press the "Stop/Clear" button, then key in a new cooking time and press "Start."

* 6 Open the door when the timer reaches the "Off" position and take out your food.

Thanks.

This is a list of possible causes. 1. Defective triac or relay 2. controller is confused (pull power cord for a minute and see if it resets) 3.Defective controller or touchpad.

This is a problem in one of the damper-related components, most likely the switch, but it may also be the motor or the damper door.

The damper is a door inside that opens or closes depending on cooking mode. If the controller senses that the door is not in the right position, it will shut down the cook cycle.

Sharp models stop after 59 seconds.

See the attached sample photo for a typical damper assembly.

We have the applicable service manuals for these models and have uploaded it to our site here and here to help you. You will need the free Adobe Reader to view or print it.

If a switch is all you need, we offer a universal type for only $5 postpaid which you can order here.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

This is a problem in one of the damper-related components, most likely the switch, but it may also be the motor or the damper door.

The damper is a door inside that opens or closes depending on cooking mode. If the controller senses that the door is not in the right position, it will shut down the cook cycle.

Sharp models stop after 59 seconds.

See the attached sample photo for a typical damper assembly.

We have the applicable service manuals for these models and have uploaded it to our site here and here to help you. You will need the free Adobe Reader to view or print it.

If a switch is all you need, we offer a universal type for only $5 postpaid which you can order here.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

We are all in the same boat. It is very annoying. I think it is a sensor tripping.

If the time from START to shut off were EXACTLY 3 minutes, then I would suspect a sensor problem or corrupted code in the microprocesor. But since that time sounds more random, it sounds heat-related, and I would suspect a failing magnetron or a failure in the cooling system. If the cooling fan is not running while the oven is cooking, this failure can occur, and can destroy the magnetron in short order. A cracked magnet (has the oven been dropped?) or other problem can cause a magnetron to go bad and overheat. If it's under warranty, I would definitely contact the manufacturer (see your manual) to find an authorized local servicer. If it's out of warranty, I would balance the cost of a new microwave versus a bench diagnosis fee of $25 or more or an in-shop repair cost of maybe $80 or more.