the error 'Codes'

How long does the internal computer keep the error codes in memory? Does it keep a x-amount of error codes, and when another is thrown is pushes the oldest one out? Also- how can the memory for the error codes be refreshed/erased. I'd read on here once before that by disconnecting the battery for a period of time would do it. I've also read that it requires the software available at the dealer. (say it aint so!)

I think it stores them for 30 restarts.

remove them by disconnecting the battery.