upgrade iphone 3Gs to 3.1.3 without upgrading baseband

here is the procedure on how to upgrade your iPhone 3Gs 3.1.2 with baseband 5.11 to 3.1.3 without upgrading the baseband.

1. first make a modified 3.1.3 ipsw using the instructions provided here.
2. shift-restore on your itunes and use the modified firmware that you created.
3. you will come up with error 6 on itunes as shown by the image below.



3a. then you phone will be in recovery mode as shown.



4. now open ireb and press fix recovery loop, then choose iphone 3Gs.



5. Now your phone will be back to normal as shown below.



6. here's the screenshot of the phone with 3.1.3 version and 5.11 baseband.



download ireb here.