An LG microwave that goes through the motions without heating is describing a fault in one place. The low-voltage side, the control, the light, the turntable motor and the fan, is working, because you can see and hear it. The high-voltage side, which turns household current into the energy that heats food, has stopped. Understanding that split matters, because it separates the things worth checking from the thing you must not touch.
Rule Out the Settings First
A microwave set to a low power level runs its magnetron in short bursts and a bowl of soup can come out barely warm after a minute. Check the power level is at full for a test. Make sure a Defrost or Keep Warm program is not running, and that the clock and timer functions have not been started instead of a cook cycle. Then run a controlled test: a cup of water, one minute at full power. If it is hot at the end, the oven heats and the complaint is about a recipe or a setting. If it is cold, continue.
The Door and the Outlet
The door operates several switches, and the oven refuses to heat if any of them does not close fully, even when the door looks shut. Look for a crumb or a bent latch hook, and close the door firmly. Confirm the outlet is live by plugging a lamp into it, because an over-the-range unit sits in a cabinet on its own outlet that is easy to forget. Check the breaker. A unit that is completely dead rather than running without heat is a different fault.
What Has Failed Inside
If the settings are right and the door closes, the fault is in the magnetron, the high-voltage diode, the high-voltage capacitor, the transformer or one of the door switches. Each has a characteristic sign. A loud hum during cooking with no heat often means the magnetron or diode. A microwave that heats briefly and stops, or trips its breaker, points elsewhere. The technician identifies which by measurement, not by symptom.
Why the Cabinet Stays Closed
The high-voltage capacitor inside a microwave stores a charge that can stop a heart, and it keeps that charge after the plug is pulled, sometimes for a long time. Discharging it safely is a specific procedure with the right tool, and it is the first thing a technician does before touching anything else inside. Videos that show a capacitor being shorted with a screwdriver are showing a gamble. There is no owner-level task inside a microwave cabinet, not a fuse, not a bulb, not a switch, that justifies opening it.
Stop Using It If
Some symptoms mean the microwave should stay unplugged until it is looked at rather than being tested further: sparking or flashes inside the cavity, a burning or electrical smell, a door that will not stay latched, a cracked door glass or a damaged door seal, or a unit that runs with the door open. Each of those is either a fire risk or a risk of microwave energy escaping the cavity, and neither is worth one more cup of water.
Repair or Replace
Over-the-range microwaves are mounted to the cabinet and wall and vented through the cabinet, so replacing one is more work than lifting a countertop unit off the bench. That changes the arithmetic in favour of repair on a unit that is otherwise sound. Book an LG microwave repair with the model number from the plate inside the door frame, what the water test showed, and whether the oven makes any unusual sound while running.