i found this at llamma's website
"21 - anywhere - This error says that the machine was booted to display a error, basically someone told the machine to reboot (or launch a xbe) with this flag, and the error code just means its been rebooted by the flag
In closing:
21 was the highest number I could find, I don't think any thing is past that. And for the unknown ones, I couldn't find any reference to them at all, I don't believe they are used. "
for an entire list
go here
http://llamma.com/xbox/Repairs/expla...rror_codes.htm