LG microwaves rarely show fault codes at all, which is why F3 alarms people when it appears. LG's article on the code describes it as a key problem: the control is seeing one of the touch-panel keys as pressed continuously, or as shorted, and it stops responding until that clears. Nothing about the cooking side of the oven is implicated. The magnetron, the turntable and the door switches are all fine as far as the control knows.
Why a Key Sticks
The touch panel is a membrane with conductive traces separated by a thin gap. Moisture that gets behind the membrane, from steam rising off the cooktop under an over-the-range unit or from cleaning spray applied directly to the panel, bridges the gap and holds a key down. Grease film does the same more slowly. A membrane that has warped from heat can also rest against its contacts. Occasionally the ribbon cable between the panel and the control board loosens or corrodes and produces the same reading.
The Owner Steps
Unplug the microwave, or switch off its breaker if it is a built-in with no reachable plug, and leave it off for several minutes so the control fully resets. While it is off, wipe the panel with a cloth barely damp with mild detergent, then dry it thoroughly, and never spray cleaner onto the panel itself. Look at the panel in good light for a bubble, a crease or a key that sits lower than its neighbours. If the unit is over a range, think about whether steam from a pot has been rising straight into the control area, and use the hood fan when boiling. Restore power and press each key once. If F3 has cleared and the keys respond, moisture was the cause and it may return the next time the panel gets wet.
Where the Owner Job Ends
If the code comes straight back, or comes back every few days, the membrane or the ribbon connection has failed and the panel assembly needs replacing. That is technician work, and not because the panel is difficult. The microwave cabinet contains a high-voltage capacitor which keeps a lethal charge for some time after the unit is unplugged, and the control panel wiring runs directly past it. There is no owner-level reason to open a microwave cabinet, and a stuck key is not one.
Ruling Out What F3 Is Not
F3 is not a door fault; a door that is not closed produces no code and simply refuses to start. It is not related to the F3 that appears on LG ranges, which belongs to a different code scheme entirely, and looking up the range meaning gives you a story about an oven sensor that has nothing to do with a microwave.
Booking a Panel Replacement
Say whether the code appears at power-on or during a cooking cycle, whether it clears after a reset and for how long, and whether the microwave sits over a range. Include the model number from the plate inside the door frame or on the cavity wall. Book an LG microwave repair and leave the unit unplugged until then rather than pressing keys that the control cannot read.