There are 4 thermostats (aka TCO = Thermal Cut Out) in the GE JVM1440BH04 The schematic is under the cover in a black envelope to the top left of the unit if you face the door.
Check the fuse first. You can remove the front grate, then the numpad on the right. The fuse is right behind it. It's a 250V 20A Class F fuse.
There are four (4) TCOs or Thermal Cut Out thermostats in this puppy:
1) There is a thermostat behind the button panel to the left, facing down. If blown, it breaks the power coming into the unit and will cause a no power situation.
2) There is another TCO or thermostat near the top lamp, to the left, in a small cavity. That one also breaks power to the unit and will cause a no power situation. It's a 100 C / 0 C thermostat. This was the one that blew on me after I left the stove running (for heating in a black-out) under the microwave.
"Cavity TCO" or "Thermostat" part number 61 on the schematics, part number PS239319
3) There is yet another power-breaking TCO or thermostat in the cavity behind the fan that cools the microwave coils. You have to remove the sheet metal on the right of the microwave to access it. This one also breaks power and will cause a no power situation.
1), 2) and 3) should test out to be 0 Ohms with a resistance meter (Digital Multimeter) if working correctly.
4) There is a thermostat which normally does not let current through it. It is mounted in a vertical position to the back wall behind the front panel on the right. This one is used for the sensor reheat function apparently and will close when the temperature in the microwave reaches approx. 60 C.
4) should have infinite resistance with a resistance meter under normal circumstances.
Had trouble isolating the problem, so I had a repairman identify the problem, then I ordered the part and replaced it myself.
Removed screws holding the grill in place. Removed air filter, removed screw holding plate under the filter in place. Reached in under fan cowling to find the thermostat cut-out. Disconnected the two wires. Put the new thermostat in place. Replaced screws plates and filter in reverse order.
