OK, perhaps we're talking about different things, let me clarify. I'm referring to the number in the upper left hand corner when you see the "Your Xbox needs Service" screen.
Case in Point: I have 6 Xbox's (all v1.0), in my rat's nest of a work area, that have been fixed in the past 3 days. Only one of those six was able to generate an error code as to its problem (error 16 to be exact). Now, after installing the xenium and booting from an x2 4981 bank, an error code was generated for each and every one (most of them being error code 6). So, booting from the original BIOS on these Xbox's does not generate a number, simply "your Xbox needs service".
These are early production boxes from late Oct. 2001 and early November 2001. I believe the one that generated the proper error code number was dated July 30, 2002 (I just checked and verified the date).
I've talked before (on these forums) about some v1.0 boxes not being able to generate an error code number (with the retail BIOS of course) and have received feedback from other individuals stating the same. It seems this is an issue related to early production Xbox's only.