I read from other post about lossing the OBD2 when standalone computer are installed. what's OBD2?

 

OBDII
On Board Diagnostics II
The ability for the computer to self diagnose an emmision related problem.
In 1996 the U.S went to a standard "plug" so that all vehicles could be run thru self tests with one kind of scan tool.
With a standalone, There is complete programming available to the end user. That makes emmision watchdogs very upset.
The OBDII on current vehicles maintain closed loop as long as possible to reduce harmfull emmisions from the tailpipe.
Standalones dont have an emmision feedback system

 

BTW here is good link to read if you really want to understand
http://www.batauto.com/articles/obd2explained.shtml
Hope that helps