You may want to start the troubleshooting from testing the primary interlock switch (the very bottom door switch) for continuity. Unplug the microwave. Open the control panel and remove the wires from the switch. The normal readings between the COM & NO terminals should be: with the door open - ∞ Ohms; with the door closed - 0 Ohms.
If any of the readings is incorrect or any wires or switch terminals are burned, replace the switch.
- The primary interlock switch Part number: WB24X829
Here are the breakdown diagrams and Parts for GE JVM1850CD001 Microwave - AppliancePartsPros.com