What would cause a cloud of humidity to build up in a LMV1813ST LG microwave and then blow the fuse causing no power

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

The service manual for your model is here. That and my video here will give you disassembly info.

More questions? Contact me directly here.*

* always include the full model number and FCC ID number from the label on / in your microwave

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 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

There is usually a 125V, 20 AMP line fuse inside the microwave, mounted at the point where the power cord enters the back of the unit. Unfortunately, you will need to uninstall the oven (if this is an over the range model), UNPLUG it and remove the exterior case to access. The case comes off in various ways depending on manufacture and model. If the model number you listed this question under is correct, you have THREE possibilties that may cause a complete loss of power: 1. Line fuse 2. The Oven Cavity Thermostat. 3. Thermal Cut-Out (Thermostat) for the Magnetron. The line fuse is connected in line with the power cord inside the unit and is listed as part of item 28 under the "Interior and Ventilation" parts heading. The thermostat for the oven cavity is listed as item 29 (part number 8183703) under the same parts heading. The thermostat for the Magnetron is listed as item 15 under the "Magnetron and Turntable Parts" heading. The part number is 8183698.

I would recommend you check the fuse, first. This the easiest and least expensive repair. It is also the most common. If you managed to overheat the oven any time recently, you may want to check the thermostat(s) to see if it is still good, next. They should read 0 ohms when measured out of the circuit across the terminals.

If the line fuse is blown, you can usually purchase a box of fuses at a hardware store for just a little more than what the manufacturer charges for a single fuse. 125V, 20AMP Ceramic Slow Blow fuses are recommended.

If you have any questions, or need additional assistance troubleshooting, please post back and let me know. I hope you find this information helpful. NOTE: Baked potatoes, steamed vegetables and popcorn are notrious for causing high humidity and/or overheat conditions in an oven cavity. If you find any of the thermostats tripped or blown, remember to allow the oven cavity to cool by leaving the door open between cooking items. This should only take a few minutes. Repeatedly heating items one after the other can cause high humidity and heat which may cause the thermostats to trip. Some of these thermostats reset themselves, others you need to replace.

NOTE: Make sure you take any resistance readings with the microwave UNPLUGGED and the component under test isolated (removed) from the circuit. These ovens can blow a fuse by simply sitting idle. This can occur from any power fluctuations, brown outs, voltage spikes, etc. Or, it could be the symptom of a component that may have failed. If you find the internal fuse blown and replace it, only to have it blow again, you have a more significant problem than a simple blown fuse.