Samsung microwave is not heating

Verify that the microwave is not in Demo mode.

When a microwave is in Demo mode, you can use all the functions of the microwave, but it won't heat anything. This is because the magnetron is not turned on. An easy way to test for Demo mode is to set a cooking time and start the microwave. In Demo mode, the timer counts down much faster than usual.

Inspect the door.

Make sure the door closes completely. If the door is not completely shut, the microwave may run but will not heat. Use a damp cloth to wipe the door and frame of the microwave; debris could be preventing it from closing properly.

Verify there is enough clearance on all sides.

Check the clearance to the left, right, above, and behind the microwave. If the microwave does not have enough clearance, it will be unable to vent properly. It will either not heat or have weak heat.