Dandy Designer 800 Watt Microwave

There might be a problem with door switch for your microwave to be turning off after 3 seconds and the door switch will need replacement on your microwave.

When a Panasonic Inverter Microwave shuts off after a feew seconds, it is either a bad inverter which is the quite expensive circuit board that powers the magnetron that puts out the microwave energy, or a bad magnetron, Or BOTH typically this would be a costly repair, But since it is still under warranty. You need to find a Panasonic warranty service provider in your area. If it is a counter top unit, you may have to take it to them for repair.

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...

follow this steps and fix it. God bless you

1. Main control boardIf your microwave runs for a few seconds and stops, the first thing to check is the main control board to ensure that a shortage has not occurred. If the part is defective then it will need to be replaced to prevent continual shut offs. If this is not the issue, the next step would be to check for a faulty touchpad.
   
2. Touchpad/Control panelIf the control board is not the issue and your microwave continues to shut off after running for only a few seconds, the next thing to check is your touchpad, which is also known as a control panel. Although uncommon for this issue, symptoms are usually that the display and some buttons are working properly; however, when selected some buttons are not functioning. If this is the case the touchpad will need to be replaced.
   

Step 1:

Door Switch

If the microwave runs for a few seconds and then shuts off, a door switch might have failed intermittently. Check each of the door switches for any signs of overheating, arcing, or burning. Also check each of them for continuity. Replace any of the switches that is visibly damaged or that only has continuity intermittently

Step 2:

Main Control Board

If the microwave runs for a few seconds and stops there may be a short circuit in the main control board. Check other components first, these boards are often misdiagnosed.

Touchpad and Control Panel

If the microwave runs for a few seconds and stops the touchpad and control panel might be defective. There are several other components that can cause this problem, be sure to check any other components before replacing this board.

Touchpad

If the microwave runs for a few seconds and stops the touchpad might be at fault. This is not common. Generally speaking, if the microwave display is working and some of the buttons respond correctly but some don't, the problem is with the touchpad.

High Voltage Transformer

If the microwave runs for a few seconds and stops the high voltage transformer might have an intermittent short. This is not common. Check other components related to the microwave symptom first.

Thermostat

If the microwave runs for a few seconds and stops the thermostat might be defective. Watch our video on testing thermostat

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Step 6:

use the DVOM tester and fix it. God bless you

Click on : Microwave bulbs

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see this causes and fix it. God bless you
If the microwave runs for a few seconds and stops the thermostat might be defective. Watch our video on testing thermostats.
If the microwave runs for a few seconds and stops the thermoprotector may have tripped. This is a safety device to prevent the microwave from overheating. It can be tested for continuity to see if it's blown.
If the microwave runs for a few seconds and stops the high voltage transformer might have an intermittent short. This is not common. Check other components related to the microwave symptom first.
If the microwave runs for a few seconds and stops the touchpad might be at fault. This is not common. Generally speaking, if the dishwasher display is working and some of the buttons respond correctly but some don't, the problem is with the touchpad.
If the microwave runs for a few seconds and stops the touchpad and control panel might be defective. There are several other components that can cause this problem, be sure to check any other components before replacing this board.
If the microwave runs for a few seconds and stops there may be a short circuit in the main control board. Check other components first, these boards are often misdiagnosed.
If the microwave runs for a few seconds and then shuts off, a door switch might have failed intermittently. Check each of the door switches for any signs of overheating, arcing, or burning. Also check each of them for continuity. Replace any of the switches that is visibly damaged or that only has continuity intermittently.

In most cases when the microwave stops @ 23 seconds it's the Magnetron which has failed, and is shorting to earth.

Either that or the magnetron isn't getting the right signal from the inverter board. It gives it 20 seconds to get the right signal or it shuts itself off.

I found an envelope with troubleshooting info inside the microwave on the left side behind the front vent. A troubleshooting flowchart was included which indicates that this problem is caused by the high voltage capacitor. I got mine out pretty easy and ordered one from appliancepartspros.com for $55 (which includes expedited shipping. I hope this info helps.