Error F3C on Whirlpool convection microwave
OK. I fixed my oven today. Here is how it happened:

First, I found my tech sheet, using my fingers instead of my old eyes, I located the black poly bag containing the data sheets for the microwave. (It is located behind the front grill, in the center next to the charcoal filter, in a slot also made of black plastic.)

From the technical data sheet, I found that my error code was for the Forced Convection Temperature Sensor, and that this is a thermistor, with a room temp resistance of about 230 K-ohms. It is wired with a red wire on one side, and white on the other.

After uninstalling the oven, and removing the outside cover, I located the thermistor on the left-hand side next to the heating element. I disconnected it from the control panel, and checked the resistance and found it to be correct. I checked it further and found that the white wire was shorted to the chassis.

After further examination, I found where the white wire from the sensor was incorrectly routed, and had been pinched during manufacture between two pieces of sharp sheet metal. The location of the short was just as the wire exited a hole near the right hand side top next to the HV capacitor. The wire seemed good, so I just routed it correctly, and placed some electrical tape around the worn insulation.

The convection part of my oven is working again!