Runs like normal but won't heat anything. Microwave randomly turned on by itself. It ran for a while without anyone noticing. Now it will turn on but it won't heat heat anything up.

A microwave oven works on the principal of a high power high frequency radio transmitter. The device that emits the microwave energy is a tube called a magnetron . When you run the microwave empty with nothing to absorb the microwave energy it is all reflected back to the magnetron causing it to overheat and destroy itself or cause failure in the High Voltage power supply due to excessive current draw.This is caused by standing waves.

What was the point of running it empty?. Looks like you, as many users out there, never read instructions for any thing. No offence!
First let me explain what a MICROWAVE does, then you will understand the damaged you have caused to your unit.
A microwave is an electromagnetic wave (radiation) with a wavelength in the range 0.001 to 0.3 shorter than a radio wave but longer than an infrared. They are used in various scenarios; industrial processes, communications and heating.
When these waves enter the food or liquids they are absorbed by their molecules which turns into an increase of the temperature of the items exposed.
If nothing is exposed to absorb these microwaves, they will be reflecting in all the walls of the cooking chamber and eventually entering the grills (of the intake and exhaust vents) damaging the the electronic components exposed to these waves.
Hope that helps!

Al

Since the light and fan work the main board or the processor is working .Now identify the parts by clicking this link: http://www.espares.co.uk/advice/microwave/a/5/141/identifying-microwave-parts.html

As there is no heating please do not switch on unless you have confirmed the fault. It is possible that the Magnetron is faulty, the temperature cut off is faulty or the main capacitor is short. It is also quite possible that insect can get into the oven and create soft spots. When you switch on these will create a short and make the voltage jump and in the long run damage the magnetron.
So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off. Check the wave guide covers, if food had deposited
then it must be checked or replaced.

See link: http://www.espares.co.uk/advice/microwave/a/5/1146/how-to-replace-a-microwave-guide-cover.html

Now keeping clear of the microwave, switch on, use a cup of water inside to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor can be faulty. replace with same type.
If there is heating without any issues, you can use the microwave safely

Thank You for using FIXYA.

If there is no heating please do not switch on unless you have confirmed the fault. It is possible that the Magnetron is faulty, the temperature cut off is faulty or the main capacitor is short. It is also quite possible that insect can get into the oven and create soft spots. When you switch on these will create a short and make the voltage jump and in the long run damage the magnetron.
So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off.
Make sure that the program controller is working if not check power - step down supply, the pads are responding- if not faulty pads- or the door switches, if contact is not through the microwave will not work.

Now keeping clear of the microwave, switch on, use a cup of water inside to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor can be faulty. replace with same type.
If there is heating without any issues, you can use the microwave safely

What fuse did you change? The fuse on the wiring that leads to the microwave needs to be intact, then the fuse in the Microwave itself needs to be checked. Usually there is a plug where the microwave is plugged in, in the cabinet above the microwave, you can take another small appliance or a power tester to make sure power is available in the outlet where the microwave is plugged in. If the power is on at the outlet then you'll know that it is the microwave itself that has turned itself off for some reason. You can get manuals for these ovens at this website

The most likely problem is the magnetron that heats the food, the other less likely is the high voltage transformer that energizes the magnetron. To replace either part with labor would be as much if not more than a brand new microwave oven.

Magnetron is blown.
This high-energy microwave-transmitting device hides behind a small plate on side of oven.
The waveguide sticky-out part looks like a small tube.
Normally, water molecules absorb the energy, heating the food.
Without an absorbing 'load' inside the oven cavity, the high-frequency, high power energy bounces back into the magnetron and it destroys itself. (Magnetron Hari-Kiri).
In a nutshell, tech must renew magnetron.
Then iInstruct family members to never run it empty.

don;t worry;
all you need to do is lubricate the fan bearings with light machine oil and make sure the fan is not hitting anything

This sounds like the unit is defective and if I were you I would return for replacment!

We have had problems like this before. Most likely the smartboard is defective.Check with manufacture to see if there is a recall.You should be able to find them on the web.