Diagnosis= failed thermister (temp probe inside the oven cavity) The part number is WB23X5340 and it lists for ~$72.00 at your local appliance parts place.
Bad news is that you have to pull the oven out of the wall to do this repair.
Good news is that it's not difficult to do.
TURN THE POWER OFF AT THE BREAKER BEFORE YOU PROCEED!
Otherwise you'd look like this...
All you need is;
1.) Phillips screwdriver
2.) 1/4" socket/nutdriver
3.) Ratchet if you don't have a nut driver
4.) 36 quart Igloo ice cooler (like what you'd take to the lake or the beach)
5.) Sturdy wire (smaller than a coat hanger, preferably)
Did I mention TURN THE POWER OFF AT THE BREAKER BOX?
Now we're ready for surgery.
Open the oven door and look where the oven meets the wood. You'll see a couple of screws there... remove them. Now the oven is ready to pull.
Place your Igloo cooler under the oven. Slowly pull the oven from the cavity until you have it out and sitting on the cooler. (don't sweat it... the oven is light. maybe 75 pounds or so.)
Now that you have it out, open the oven door and remove the oven racks. Look inside (may need a flashlight) and locate the temp probe, there are 2 screws holding it in place. Remove those 2 screws and set them inside the oven cavity, you'll need them soon.
Now go to the back of the oven (slide it if necessary) and remove the back panel. (*Here's a tip... some of these GE units? All you have to do is remove one screw to access the temp probe, so look carefully... don't go nuts here. Sometimes the whole panel/panels DON'T have to come off)
You'll see the temp probe wires... they are 2 small white thermally insulated wires coming from the cavity to the wire harness in the back. Disconnect this harness and pull the old probe out from the front of the oven cavity.
OK... now to get the new probe installed... See that plastic thing on the end of the new probe? It's a pain to get through the cavity where you'll be plugging it into. It's best to have help. but it can be done alone (I've never had help, it's just harder)
Take the sturdy wire (from the tools list) and make a small hook at the end of it. Poke it through the hole that the old probe was stuck through. Now hook your new probe into the hook. Now go to the rear of the oven and gently (did I say gently?) pull it through to the back of the oven.
Now that you have it through, connect it to the harness. Now all you have to do is reinstall the rear panel/s and the oven. No biggy. Just take care not to scratch your wood trim... line it up before lifting and installing.
All in all about an hour's worth of work. I know it sounds like alot more but I could have done this in less time than it took me to type it.
Congrat's on your first major oven fix! Let us know how it turned out.