How to Troubleshoot & Replace Lg AP4437957 / 3B73362F Microwave Door Switch

The fix was simple due to the comprehensive wiring diagram and explanation of role of switches that is found on a piece of paper inside the control panel. The culprit was the primary interlock door switch that failed in a curious way. With door open the switch button is fully extended and contacts are open (OK), but as the button on the switch is depressed it first goes closed (OK), but if the switch button is fully depressed it goes open again (not OK) in this particular defective switch. You need to physically remove the switch to monitor this type of failure, so it was somewhat tricky. The new switch does not go back to open if the button is fully depressed with a closed door, so repair was quick and easy with a new switch. 

Bought new micro switch from ApliancePartsPros.com. Ordered one day and got it next. That is a record. I think the guy carried the switch to carrier and told them to carry it to me special. It was not next day either it was regular mail. I repaired the microwave the same day. Thanks for prompt service. 

Viewing U-tube videos, I determined that my problem was the same as many others'. I searched for the micro switch using the model number of the microwave and found two possible switches that it may be. I ordered both of them since they were less than $3 each and they were delivered the NEXT DAY! I opened the case using a security torx screwdriver, found the switch that was stuck in the closed position, removed it, replaced it and reassembled the microwave. Start to finish repair in 20 minutes. It works great! 

I figured out which switch was bad and replaced it. 

Replaced micro switch-button on switch did not spring out. Extremely fast delivery of part-ordered part around 4pm/arrived 11a next day 

Replaced the door switch. Push-on wire connectors and snap the switch into locked position. 

The unit was not heating and the plate rotation was not working, so I followed the advice on this forum to inspect the door switch. I took the whole unit down from the wall, but I didn't need to in order to access the switches behind the button panel. I was able to open up the front panel and saw the 3 door switches inside the unit. I saw that the top switch was depressed while the other ones could be moved and had the click confirmation. I ordered the Microwave Door Switch to replace the top switch. I removed the faulty one and installed the new one. I tested the microwave and it is fully operational. 

Replaced the failed door micro switch that kept the microwave from running. 

The microwave "ran" fine but did not warm food. Suggested fixes were magnetron and/or door switches. Since I had to open the unit, I opted to replace the magnetron and all switches at the same time. All in it was about $85. All in all, it went very smoothly. I suggest a team lift to get the unit down and keep track of your screws! Unplug microwave. Remove the screws (2) for the control panel. Lift the panel straight up and then carefully tip the top out. Remove all the screws to remove the top/side panel. The quantity below is based on memory. Apologies for any inaccuracies. ~3 screws for the top vent ~5 screws on top of the panel ~6 screws on the bottom (3 per side) ~10 screws on the back (3 per side, 4 on top) Once the panel is loose, slide the panel back as you rotate the back bottom corner up and out. Discharge the capacitor. Holding an insulated pair of needle nose pliers, touch one jaw to the positive and one jaw to the negative terminal. Repeat a couple of times to make sure fully discharged. Unplug the magnetron and remove the mounting screws (4). Mount new magnetron and plug-in connector. Unscrew the top and bottom mounting screws for the door switch bracket. Slide the bracket straight up and pull out. Replace door switches. The top and bottom are the same model switch, the middle is different. Remount door switch bracket. Replace top/side microwave panel and screws. Replace control panel. Remount microwave. Drink beer. 

Watched YouTube. And repaired it. Easy peasy!