Here is the parts breakdown
Replacement parts for FRIGIDAIRE FEFL78CQA Frigidaire/electric Range | AppliancePartsPros.com
Here is the wiring diagram
When I turnon the oven or broiler the control displays operates indicating that the unit is functioning.
If you mean that it comes ready or shows a temperature higher than set point , it could be the temperature probe (Item 37 in Section 2)
Here is a good link with info on how to check it.
Check out 7, 8, 20 and 21.
If the probe is OK then it could be a bad connection, a wire burned off or the control board/clock (Item 20 in Section 1).
The following may also come in handy.