Megger PAT 420 Error 83 fault

[Mad Axeman]

As it's quiet in work I've been looking through all the faulty equipment that we have. There's a Megger PAT 420 PAT Tester (Ok, that should be ISITEE but it just doesn't sound right) that has a fault. When you power it up it goes through it's self check. The little spinning wheel symbol comes up on the screen then it stops with the following message :-
ERROR 83 - INPL_V failed.
You can't get past this no matter what you press. I can't find any reference to this anywhere. Anyone got some tips?