If you were running the 06.15.00 baseband on your iPhone 3GS and accidentally updated to the stock iOS 5 firmware, your baseband is most likely in an inconsistent state. Meaning you will either be unable to connect to a cell phone tower or you will have troubles maintaining your 3G connection. This is because once you update to the 06.15.00 baseband, you can cannot downgrade it to a normal iPhone baseband without running into major troubles.
In order to fix this error you will need to reflash the 06.15.00 baseband on your iPhone 3GS using RedSn0w. After reflashing the 06.15.00 baseband using RedSn0w you should no longer have any troubles with your baseband. The Dev-Team noted, in their changelog for UltraSn0w 1.2.4, that you can use any version of RedSn0w past version 0.9.9b4 to reflash the 06.15.00 baseband to fix any errors associated with updating to the stock iOS 5 firmware with on the 06.15.00 baseband.
Some iPhone3GS users with the 06.15 baseband may have tried to install iOS5 using a stock IPSW (even though you should never ever try to use a stock IPSW if you’re an ultrasn0w unlocker). If you did try this, your baseband is probably in an inconsistent state, and you’ll need to reflash the 06.15 baseband again (using redsn0w). Be very careful if you use redsn0w to reflash the iPad baseband — don’t interrupt the process! And please avoid using stock IPSWs in the future. Unlockers should never go near stock IPSWs. If you need to use redsn0w for any of the above tasks, please make sure it’s version 0.9.9b4 or higher.
In this how to guide from iJailbreak.com you will learn how to reflash the 06.15.00 baseband on your iPhone 3GS running the iOS 5 firmware.
Note: As I do not have an iPhone 3GS I am not able to verify if the steps in this how to guide are exactly in the right order. Please let me know if you notice any errors following this how to guide.
How To Reflash 06.15.00 baseband
Step 1) – The first thing you must do is download RedSn0w 0.9.9b6 which is compatible with both Windows and Mac OS X from the iJailbreak Downloads Section.
Step 2) – When RedSn0w has downloaded you will need to launch RedSn0w.
Step 3) – Once RedSn0w is up and running you will need to click the Extras Button followed by the Select IPSW Button. Upon clicking the Select IPSW Button you will need to select your iPhone 3GS’ iOS firmware file (the firmware it is currently running… most likely iOS 5). If you do not have your iPhone 3GS’ firmware file please download it from the iJailbreak Downloads Section.
Step 4) – After you have selected the iPhone 3GS iOS firmware file you will need to go back to the main menu and click the Jailbreak Button.
Step 5) – You will now need to put your iPhone 3GS into DFU mode. Read the instructions and click the Next Button to begin the DFU mode process.
Step 6) – Once your iPhone 3GS is in DFU mode you can select options to customize your Jailbreak. You will need to select the Reflash 06.15.00 baseband option and uncheck the Install Cydia option.
Step 7) – After you have selected the Reflash 06.15.00 baseband option, click the Next Button and RedSn0w will begin re-jailbreaking/re-flashing your iPhone 3GS’ baseband.
After the process is complete you should notice your iPhone 3GS should function correctly and any 3G connection issues/service issues should be fixed. Please let us know if this how to guide on reflashing the 06.15.00 baseband on your iPhone 3GS worked for you in the comments section below…