So you’re looking a 3.03 PSP Downgrade? well this article may help you out.
First and foremost, you need an unpatched copy of Grand Theft Auto Liberty City Stories.
There is no other way that you can downgrade any PSP over 2.81 without it. Now for some reason people don’t seem to appreciate this fact and will spend hours upon hours searching the Internet for other ways to downgrade with out the GTA. Let me save you time, there is no other way to downgrade any PSP above 2.81 with out the unpatched GTA.
In late 2005 a vulnerability was found in the way the PSP version of GTA: Liberty City Stories processes saved games. In December 2005 software was developed to execute unsigned code on PSPs with firmwares 2.00 through 2.60. In April 2006 firmware 2.70 was released and patched the exploit, however, as of January 25, 2007 it was discovered that Sony did not completely patch the exploit, and unsigned code may be run on 3.03 firmware, a 3.03 downgrader was released 3 days after the exploit was found. Also, new copies of GTA: Liberty City Stories patched the exploit as well, preventing it from being executed on other firmware versions. Since then...