GE Spacemaker XL 1800 Model JVM1851BD002

I'm betting your base model number is JVM1860, JVM1861, JVM1870, JVM1871, JVM3660, or JVM3670.
That's the automatic louver you're having trouble with. There are micro switches in the louver assembly which tell the controller when the louver is open or closed.
If one of those switches or the louver motor fails, the louver will not work correctly.
Did your cooktop lights recently fail?
Sometimes the surge from them shorting will cause damage to the circuit board, which can be repaired.
There should be a "mini-manual" (tech sheet) hidden inside the unit behind the control panel or hidden on the left side behind the grille, which is very helpful when troubleshooting, testing, and locating components.
There is a video here showing how to remove a typical over the range control panel assembly in under 5 minutes.
You can find helpful exploded view diagrams and order parts by entering your full model number from the tag on the oven here.

My guess is the magnetron is bad. It is rarely the transformer

https://www.manualslib.com/products/Ge-Jvm3670wf-Profile-Spacemaker-Xl-1800-36-Microwave-Oven-1842926.html

Click link to download manual

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.

i have a ge 1800 model and it cooks fine but after awhile it shuts down and in about a hour it comes back on and ask you to set the clock.....

It sounds like the main transformer has failed. It would be cheaper to replace the whole microwave than repair this one. Sorry to be the bearer of bad news. Hope this helps. Good luck.

The first thing I would suggest is a hard reset. Unplug the unit for a minute or so. Then plug it back in, set the clock and try again.

If that doesn't work, leave it unplugged for about 5 minutes.

check fuse inside unit microwave has a fuse around 15 to 25 amps get at local hard ware store u should discharge capacitor before installing fuse not nessary in microwave

if there is no plate to unscrew in front of the bulb , then the only way it can be changed is to open the microwave
WARNING: HIGH VOLTAGE CAPACITOR INSIDE WILL STILL BE HOLDING HIGH VOLTAGE
if you are not familiar with this then don't attempt the repair!
then you will need a technician

electech