How to Troubleshoot & Replace Lg AP4440371 / 3B74133Q Time Delay Fuse

Removed front control panel, which provided fuse access. Removed and replaced fuse 

The high-voltage capacitor shorted and it blew the main fuse (20A) causing a loud popping sound. I replaced the capacitor the fuse and the related high-voltage diode (I think the diode was stressed during the short condition). During troubleshooting and prior to the repair I checked all the switches, the high voltage-transformer, the diode and the capacitor which had a dead short of less than 1 ohm. Thanks ApplianceParsPros.com for having these parts handy. 

I have a multimeter so I was able to check the fuse per website recommendations and it was blown. However, the capacitor on the microwave was also original so I elected to replace the capacitor, fuse, and the ground protect diode. The repair was very simple (making sure the capacitor was discharged before replacing) and the microwave is now up and running again. Excellent trade to buy $40 worth of parts rather than a new $400 microwave. One nit - Kenmore puts a screw in the back that requires a special screwdriver head to get it out, probably to keep people from taking the cover off the microwave and working on it. I used locking pliers to grab the head of the screw and twist it out. When I put the microwave back together again, I used a normal screw so I wouldn't have to mess with that special screw again. 

Replace Thermostat on top of cavity. The thermostat is attached via 2 bending sheet metal tabs. Ours blew because the two wires connected to thermostat shorted, connectors melted. The short was caused by buildup of grease under thermal wire cover, shorting both leads. The unit is used heavily. This is the first thermostat to go. I had to replace wire connectors that connected to thermostat as well since the connectors were melted as well. Helpful hint. The grease build up was probably caused by obstructed air flow from the stove top through the exhaust vent. BE CAREFUL. The exhaust vent on the back of the unit has a flap that opens up when the fan is on. There are tabs on the top of the vent frame that are bent down, so that when the flap opens it is blocked from opening 100%, and instead only opens about %40. This obstructed flow, in my opinion, reduces air flow rate, which allows exhaust to back up and collect, as well as thermostat and unit to overheat. Solution: Bend the tabs up on the back of the exhaust vent to allow the door to open fully. Double check if there is another vent on your wall to the exterior, and make sure it fully opens as well. 

Replaced thermostat on rear of oven and that was it. I tested continuity after removal and failed, and also malformed so you could tell was bad. Thermostat on top of unit was good. Recommend buying both when purchasing as found several posts of either failing although the rear was more common - seemed to be different parts but not sure if interchangeable. We used the oven earlier that day at 500 degrees so assume that's what cause was. We've run self clean in the past and may have been contributing factor. 

You tube and other sites said it was one of the door switches, called micro switches...so I went to AppliancePartsPros as I always do, and ordered ALL switches and one fuse for less than $21...then I used You tube to figure out how to remove the control panel to expose the switches...I used a magnifying glass and bright light to match new to old so as to confuse switches as they all three looked alike..installed, while unplugged of course, in 30 mins and had my $500 micro working and saved a bundle...the bad micro switch has a spring loaded orange or green button that gets pushed in..it should always click back out..mine was stuck "in", so if you can see/test the button by pressing it and it isn't stuck in, then may be ok...but for price, i replaced all switches with new...be careful pulling off wire clips, pry with flat screwdriver and watch that the metal clip slides back inside the plastic covering before you re-plug it onto switch..use mag glasses and good lighting and fine point needle nose to do work. 

After I researched what could be broken, I decided to buy the most common parts listed. I put the Magnetron in first, and the oven worked. So, I have the fuse and capacitor for the future if needed. Doing it myself saved well over $100 for repairs or simply buying new. 

When using "Timer" function turned on unit to cook,empty by mistake! Removed over range unit, and removed cover. Magnetron had pieces of permanent magnet hanging on outside. Ordered parts Received parts (2days) Replaced parts and we have our microwave back, Very simple! About 15min. to fix and 1/2hr. to clean inside unit. 

After researching the most probable causes, I purchased the top two likely parts (the main fuse & thermistors/thermostat). After reviewing several videos of generic over the range (OTR) microwave ovens, I attempted to remove the requisite parts to gain access to the fuse (only after unplugging the m/w oven from the wall outlet!). I tested the old fuse for continuity using my multimeter (sound is good, no sound is not good) and confirmed it was bad. I checked the replacement fuse, just to make sure it was good before installing it, in the same manner. It was good. The access was not as advertised and the directions on the LG website were not helpful. After some maneuvering, I replaced the fuse, along with the parts I had previously removed. I then plugged the cord back into the wall outlet and everything is working! With some patience and a $5.00 fuse, I saved $200.00 by doing it myself (and didn’t break anything or injure myself in the process). Not too shabby. 

I ordered an replaced the Diode and the thermostat; Fuse was ok .This was a Kenmore, I had to remove it and open the case to be able to access these parts.