What is the most commonly occurring fault with this type of electrically driven gyro? I have an examination tomorrow about advanced avionics systems (EASA module 13) and we are required to troubleshoot a fault with the compass system. What is the most commonly occurring fault with a gyro compass system and how is it detected or identified?
Some idiot using magnetized a read by flux valve. 
Gears  slipping. Sync knob sticking in.
Leaving it in DG mode.so it drifts

Not correcting for Magnetic Variation changes unless you fly along an isogonal