VID'S GUIDE TO REPAIRING HOME POPCORN MACHINES:
At some point you will note that the Kettle does not heat up, or it does heat up, but only 10% of the kernels pop.
When you call the company, they will tell you that your kettle needs replacing at $90 to $175.
First check that you have voltage at the plug the pigtail plugs into on the roof of the machine when the Heat switch is engaged.
On inexpensive machines, the heating element is braised to the kettle itself - but the element almost never burns out in a home setting.
After checking that the heating element is conductive with your meter, you now know that it is the thermostat or the high limit switch.
If the thermostat has the tiny set screw adjustment in it, you can often just give it a turn to clean the tip and then put it right back where it was set. If that does not fix it, replace it ($5 part).
If the thermostat/limit switch looks like a metal diode, just replace it (.50 cent part).
You can put the disassembled kettle in a vice and test the whole thing on your bench with a laser thermometer, but be careful as you don't want to touch a 500*F element by accident.