F3 code

After the oven has been on for a minute or two I get a flashing F3 error message. I just verifierd the room temperture resistance in the oven sensor and found it to be about 1000 ohns which is what I am finding to be correct. I would appreciate any help on identifing the cause of the error code. With the oven being 20 years old I may be at the point of needing to replace it. Any idea will be appreciated.