Hi, unfortunately panasonic does not have the service manual for this model on their website anymore, but according to the manual for the NE993 FFFF Appears in the display on convection, grill or combination cooking.
Check continuity of the oven temp. sensor(if 0ohm or open crcuit) replace sensor, or it will be a faulty dpc.
The chances are that the sensor is nla.
Hope that helps 😕
Merv