Heat up the oven to 350 pull the temp sensor connector off in the back and take an ohm reading on the probe you should have around 15-16 ohms if you dont then more then likely it's the temp sensor....

The trick with temp probes is that you have to test them at various tempatures to get an accurate diagnosis don't ever go by just room temp that's why i recommended 350 then test. Good luck yeah if it checks out at 350 i would possibly suspect main board as well but a bad temp sensor can make the oven stay on for a long time. Also take in mind some units will heat up to around 75 degrees over the tempature you set then about 10-15 minutes later it will gradually work it's way down to the temp. Also some units can be between 25-50 degrees off depending on how cheap the unit is and that's just normal unfortunately.