When oven reaches a temp of 350 it flashes F3

Hello. I need the model number of your unit to help you accurately with exploded parts diagrams and part numbers. From what you describe, it sounds like you have a defective oven temperature sensor. There are only 2 things that govern temp in a range--the EOC(main oven control board) and the temperature sensor. It's connected to the main board by a 2-wire harness. Usually, there is a connector at the end of the short pigtail coming from the sensor. The contacts in the sensor can oxidize, breaking electrical connection or, the signal can be lost due to a broken wire, loose connection, etc. Most often, the sensors will have an electrical resistance of approximately 1100 Ohms +/- 10%. Usually, if the sensor checks good, the main board is the culprit. Hope I helped you!