I think thats your hour meter reading as the FC4000 uses the EV100 control system which doesn't go above code 200. FC4500 doesn't go above 300 and the new FC5200 doesn't go above 800.

The display should have a small spanner icon, is this lit when it says 978 as that would indicate a fault code. Is 978 shown on the display permanently or only when you key on/off?

What issues with the unit lead you to think you have an error code, is it functioning correctly?