3GS Downgrade guide.

[Iphone Prime]

First thing first, the untethered jailbreak for iOS 5 is not yet available at posting time (3GS OLDBOOTROM ONLY untethered)and baseband 5.16.05 can’t be unlocked. There’s no unlock available since baseband 5.14.02 was released with iOS 4.1.
However, it is still possible to unlock your iPhone by updating the baseband to 6.15.00. This is an iPad baseband and you must read the risks here. If you don’t wish to install iPad baseband 6.15.00 then this post is not for you.
Did you read and understand all the risks? Then let’s continue.
For some reason, Apple is still signing iOS 4.1, and you need to downgrade to this iOS version so that you can update your baseband to 6.15.00(if you don't have "blobs" for any other firmwires. There are other methods e.g. using SHSH blobs but that will add more steps and may not be suitable for beginners.
Warning: Proceed at your own risks!
Steps:

1. Backup your contacts, photos, apps etc and remove your SIM card.
2. Download iOS 4.1 firmware for iPhone 3GS.
3. Download redsn0w 0.9.8rc18 for Windows or Mac.(Latest one works)
4. Launch iTunes, connect your iPhone, hold down the SHIFT for Windows or Option for Mac while you click the RESTORE button in iTunes and select the iOS 4.1 firmware file you have just downloaded. If you get an iTunes error 3194
4a.Scenario 2: if you need to restore to a firmware version that is currently being signed by Apple:

You need to remove any line on your hosts file that has something like 74.208.151.171 gs.apple.com from your hosts file. You can also add ‘#’ in front of it e.g. # 74.208.151.171 gs.apple.com.
5. Start redsn0w 0.9.6rc18 that you have downloaded in step 3.
6. Browse for the iOS 4.1 firmware you just downloaded.
7. Select ‘Install Cydia’
8. Follow on screen instructions to put your device into DFU mode to begin jailbreak.
9. When the process is done, you will find that your iPhone is now jailbroken on iOS 4.1To make sure you can always go back to iOS 4.1, launch Cydia and select ‘Make my life easier.’

10. Search for ultrasn0w on Packages in Cydia. If you cannot find it, add new sources by typing ultrasn0w repo:
repo666.ultrasn0w.com (with zero, not o).
11. Install Ultrasnow 1.2.4/3
11a. Launch Cydia from Springboard (home menu). Do you have a broken WiFi?
1. Download i-FunBox for Windows or DiskAid for Mac.
2. Launch i-FunBox or DiskAid and go to /private/var/root/Media.
3. Inside Media folder create a folder called Cydia.
4. Now inside Cydia folder create an AutoInstall folder. Note: The folder name is case-sensitive so be sure to name it accurately. It should look something like this:
/private/var/root/Media/Cydia/AutoInstall
5. Now copy ultrasnow.deb [ultrasn0w_1.2.4.deb] file and MobileSubstrate [mobilesubstrate_0.9.3901_iphoneos-arm.deb] file into AutoInstall folder.
6. Reboot iPhone


I’m sure you want iOS 5 right? Let’s update it to iOS 5 and then unlock it with ultrasn0w 1.2.4.
12. Create custom iOS 5 for 3GS. If you don’t have the official carrier SIM card, Use Sn0wbreeze
13. Download the latest redsn0w from the Dev Team’s website. Start redsn0w, go to Extras > Pwned DFU. This allows your iPhone to accept custom firmware.
14.install iTunes 10.5.
15. Connect your iPhone to your computer and hold shift+restore (Windows) or option+restore (Mac) and select the custom iOS 5 IPSW you just made. If you get an iTunes error 3194,read step 4a .
16. Once the restore is done, your iPhone will now have iOS 5 with baseband 6.15.00.

However for 3GS new bootrom users you will NEED redsn0w to turn your iphone back on once you reboot

This guide was not made by me, I just copy,paste and edit to make it easier for the people of the Modmyi community,I will take no credit.

iPhone 3G/3GS

Baseband Firmware Unlock

4.26.08 3.0/3.0.1 Yes (Ultrasn0w)
5.11.07 3.1/3.1.2 Yes (Ultrasn0w/blacksn0w)
5.12.01 3.1.3 Yes (Ultrasn0w)
5.13.04 4.0/4.0.2/4.0.3 yes (Ultrasn0w)
5.14.02 4.1 None *
5.15.04 4.2.1 None *
5.16.01 4.3 None *
5.16.02 4.3.X None *
5.16.05 5.0 None *
6.15.00 Custom Firmware or redsnow method yes (Ultrasn0w)

iPhone 4

Baseband Firmware Unlock

1.59.00 4.0/4.0.1/4.0.2 Yes (Ultrasn0w)
2.10.04 4.1 Gevey Sim
3.10.01 4.2.1 Gevey Sim
4.10.01 4.3.X Gevey Sim
4.11.08 5.0 None

iPhone 4S

1.0.11 5.0 None

Iphone Prime.

[Southafrican]

My redsnow keeps closing when its "fetching ipad baseband files" ? did all the steps but then redsnow keeps closing...

[Simon]

My redsnow keeps closing when its "fetching ipad baseband files" ? did all the steps but then redsnow keeps closing...

Dont install cydia at the same time. Run redsnow once with only ipad baseband checked and then run it again with only cydia checked.

[Southafrican]

i did that... an now still when i turn it back on.. it goes to the "plug + itunes" screen....

[Simon]

What are you trying to do? What device do you have?

[Southafrican]

Hi Simon! glad to see you a Mod maybe you can help me more
I have a 3gs, jail broken, baseband upgraded iphone.. its the very first ipad baseband update.. i was running 4.2.1.. when the phone went into "plug into itunes mode" since then i have tryed almost everything to get it fixed again.... i dont know what to do anymore... iv tryed the custom restore.. i get error 1604... iv tryed the things at the top.. but as you see its not working

[Iphone Prime]

Steps above is for 4.1 since Apple is still signing it.

did you saved blobs for 4.2.1?
try restoring to stock 4.1?

[Southafrican]

nope didnt save blobs... and i tryed restoring stock 4.1 ( or what i think is stock ) and it kicks out error 1604

[Simon]

Hi Simon! glad to see you a Mod maybe you can help me more
I have a 3gs, jail broken, baseband upgraded iphone.. its the very first ipad baseband update.. i was running 4.2.1.. when the phone went into "plug into itunes mode" since then i have tryed almost everything to get it fixed again.... i dont know what to do anymore... iv tryed the custom restore.. i get error 1604... iv tryed the things at the top.. but as you see its not working

Make sure you have put the phone into pwned DFU with redsnow or iReb before trying to restore to a custom firmware. ALso make sure you have SHSH for the firmware you are trying to restore to. As stated above Apple is still signing 4.1 so a custom 4.1 is always a option.

[Iphone Prime]

when you say (kicks out) on the iphone screen its telling (showing) you to connect to itunes? if yes ues TM (tiny umbrela) to exit recovery or Recboot.

[Southafrican]

what i mean is that it gives me the "error 1604" when trying to use a custom firmware restore on itunes.. it happens when it says "phone is preparing for restore" or something like that in itunes..it hangs there and then gives me the 1604...... also im not sure if my stock 4.1 is stock or custom.. cause i now tryed using the stock 4.1 i have... to make a custom one with snowbreeze.. an it tells me its not a valid one.. so im completely lost right now an hope i dont loose my iphone... its been over a month that i been trying to get it back into working order.. i dont care what ios i run.. as long as i can get it running again...

[Simon]

what i mean is that it gives me the "error 1604" when trying to use a custom firmware restore on itunes.. it happens when it says "phone is preparing for restore" or something like that in itunes..it hangs there and then gives me the 1604...... also im not sure if my stock 4.1 is stock or custom.. cause i now tryed using the stock 4.1 i have... to make a custom one with snowbreeze.. an it tells me its not a valid one.. so im completely lost right now an hope i dont loose my iphone... its been over a month that i been trying to get it back into working order.. i dont care what ios i run.. as long as i can get it running again...

You are getting 1604 error because you are not putting the phone into pwned DFU mode.

[Iphone Prime]

Download iREB RC4.
connect your idevice to pc in DFU mode
select 3GS
watch as its being pwned
then restore your custom firmwire in itunes.

also i have notice when i am restoring any 3GS on my pc, and my host file points to the 74.208.105.171 gs.apple.com upon restoring when its complete my 3GS's stays with "connect to itunes"

i dont know if this works for others but it dose for me, i just restore my original "host file" launch itunes and restore again and after all that is done my phones ask for original sim, however i dont have original sim and i dont even live in the US, i live in the Caribbean (third world), so jailbreak and ipad bb is a must for me. in other words if you have " 74.208.105.171 gs.apple.com" in your host file remove it. and try restoring to stcok 4.1

[i.Annie]

Redsn0w's pwned DFU mode works quite well. iReb, nothing against it or anything, but I've found doesn't always work all the time. In which case if the above fails, use Redsn0w.

[qhenkart]

I am trying to install the iOS 4.1 (up from 3.x) and I am getting "the device is not eligible for this build" error. I obviously don't have the SHSH for this since I am upgrading not downgrading. I also tried doing the pwned DFU, no long. Is it possible that apple stopped validating 4.1? Does that mean I am completely screwed for unlocking?


Update #1
so I opened up tiny umbrella and started a TSS server ( I am brand new to tiny umbrella so I have no idea what I am pressing) the only SHSH I had was for 3.x, and when I tried upgrading to 4.1 I actually got the 1604 error... This was promising... so on top of that I opened up redsn0w and did a pwned DFU, together with both of those things, it seems to be working :-)

[Southafrican]

okay... i did what Prime said.. now when i push shift.. an choose the 4.1 to restore.. it started to prepare the phone... then the phone turns on... shows apple logo... then goes black again... while itunes says its preparing the iphone for restore? i put the phone in redsnow dfu mode BEFORE opening itunes... is that incorrect?
i have a original sim.. tho i am from south africa so i do need jail break and ipad bb

and i once again got the 1604... so for some reason im not putting it into pwnd dfu mode correctly? anyone have skype or something to help me sort this?

[qhenkart]

I am stuck again, this time I successfully installed 4.1, cydia and updated my baseband. I went to install ultrasnow and it installed finishing with the "reboot this device" button. My iphone needs to be tethered to boot it appears, so I boot with redsnow like I was doing before but instead of booting, I get a blank white screen that goes to a blank black screen, and nothing happens. Any ideas?

my iphone is completly bricked. I've tried everything... it is completely unresponsibe. Booting it through redsnow only makes the screen go white and then black, computer doesn't detect it until it is off. Cannot restore to any firmware

[Simon]

Just want to add to this OP, your title says downgrade a 3GS but you have how to install the iPad baseband as well. For anyone reading this DO NOT install the iPad baseband if you do not require a unlock.

[Southafrican]

im still stuck

[qhenkart]

Okay, so I had an iphone 3gs running ios 3.x, upgraded to 4.1, updated to ipad baseband, installed redsn0w and ultrasn0w, iphone was bricked

Nothing was working, I sheepishly took it to the apple store and was 100% honest, they gave me a brand new one for $75, beware! I might have been a unique case, but you never know