Jane,
The "F3" error code means the oven temperature sensor (#14 on the
break down diagram) is "open".
Locate the sensor plug at the control board and measure the sensor resistance. It suppose to be about 1100 Ohms at the room temperature.
If the reading is wrong then you have to pull the range, remove the rear cover and measure the sensor resistance at the sensor plug.
If the reading is wrong - replace the oven temperature sensor.
If the reading is correct - check the wire harness.
If the sensor resistance at the control board is correct then check the plug and if there is nothing wrong - the control board has to be replaced.
- The part number for the oven temperature sensor is
AP2831584
The control board (aka clock) is no longer available.
Here are the break down diagrams for the Thermador range
Model GSC30CVWC
Gene.