How to remove LG microwave to check fuse? Model LMV1813ST Serial: 102TRXT01402

Could be magnetron failure but cooking a "foreign" object may have led to breakdown

F4 means sensor open, You have to check the Humidity sensor.
If you wish to get some details; visit the site linked here. Pull up older posts. http://electro-medical.blogspot.com/

If there was any odor, the cloud might have come from a failed component, such as the high voltage rectifier. That could blow the fuse.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/david_29ad5d1dd86564b0

If too much current passes through the microwave circuitry, the line fuse will blow. If the line fuse blows, the microwave won't start. To determine if the line fuse is at fault, use a multimeter to test it for continuity. If the fuse does not have continuity, replace it. In addition, you should investigate and resolve the problem that caused the fuse to blow. If you don't resolve the underlying issue, the new fuse will blow too. (Caution: The microwave oven can store thousands of volts of electricity in its high voltage capacitor, even after the microwave oven has been unplugged. Due to the potential for electric shock, it is extremely dangerous to replace the electronic components in a microwave. Only a licensed technician should replace the line fuse.)

http://kitchen.manualsonline.com/manuals/mfg/lg/lmv1813st_2.html

The fuse can be replaced, but the underlying issue should still be investigated and resolved. Otherwise the new fuse will blow also.
If the microwave is not working the thermal fuse may have blown.
When a microwave stops working the problem is usually the line fuse or within the high voltage circuit; either the high voltage capacitor, diode, transformer or magnetron.

replacemotor under microwave

Go for a warranty repair anyway. It may be the magnetron, and the clicking is the big condenser discharging.
https://www.lg.com/ca_en/support/warranty
Microwave Ovens (Over-the-Range):

• 1 year parts & labour
• 5 years on the magnetron (part only)

Microwave Ovens (countertop):

• 1 year replacement warranty
• 5 years on the magnetron (part only)