I ran the microwave for 15 minutes with nothing inside. It no longer heats.

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.

It is more than likely the magnetron is fried, to replace fuse! unplug microwave oven, remove top, "several screws" look to the right to the keypad control board, look down to a printed circuit board, clipped fuse should be near by, pop it out and replace with same current rating, assemble before testing, "lethal voltages present"

Hi,
Here is a tip that will help you to figure out what is wrong with yourMicrowave Oven....

Microwave Oven Basic Troubleshooting Tips

heatman101

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.

in a microwave, there is no heating element. There are microwaves that heat up the water molecules, they make them spin fast and that's what heats things up. Running it with nothing in it, you really can tell if it's not working by that test either, again, nothing to heat up. I would put in a cup of water, set on high for 3 minutes and it should boil, if not, get a new one for the price, why mess around?

most have an internal fuse check it first if that is not it check the thermal fuse both parts run 10-15 dollars

u need to clean the cavity inside the microwave .. step to clean that .. put a cup of water nearly half filled( 30 ml ) and start microwave for five minutes .. after that just open microwave and clean from inside with clean cotton cloth .. this will surely help ..

Hi,
Here is a tip that will help you to figure out what is wrong with yourMicrowave Oven....

Microwave Oven Basic Troubleshooting Tips

heatman101
wsw

That doesn't sound good at all. Microwaves are death traps. It could have melted some metal inside. A fuse could have gone, but most likely the magnetron is shot. Might be safer to recycle and reinvest in a new microwave, especially for safety sake when dealing with controlling radiation.