oven not heating but grill still fine, same element?

Hello, I have an AEG competence single fan oven B4100-1M.
The oven has stopped heating but the grill which is on the same element and citcuit does work. I assumed that the fault would be caused by a faulty switch or element. I have changed both of these but still have the fault. I have had an electrician to test it and he is stumped because everything that he has tested works apart from when the selector is set to oven. Can you help please?