What's an SQ base unit on a Nikon D5100 and why is it faulty

[Mandat]

I had my D5100 for under 2 years when a Shutter error appeared. The camera has been sent back to Nikon for repair and they have diagnosed a faulty SQ base unit.

I've googled this and can't find exact details hence asking here. What does the SQ base unit do, and why would it have become faulty in under 2 years.

Nikon are also charging £125 for the repair, because I didn't register with them to activate the second year warranty, even though I did register for Nikon Plus at the time of purchase. Is there anything that I can say to Nikon for them to change their mind about not honouring the 2 year warranty?

[TooLateForAName]

I think that it is the thing that handles the sequence of events involved in the shutter operating. Mirror lift and drop etc.

They do seem to be available to buy if you trawl ebay.