iPhone 3GS jailbreak can be either tethered or untethered. Tethered jailbreak means that each time you restart your device you will need to connect it to a computer and run redsn0w. If you have iPhone 3GS with an old bootrom version 359.3, the jailbreak is untethered. If your device has bootrom version 359.3.2 and higher, iOS 5/4.3.5 jailbreak will be tethered.

Here's how you can detect if your iPhone 3GS has new or old bootrom version.

Method 1.

Get your iPhone 3GS serial number. Read it on the box or using Settings - General - About.

Look for 3-rd digit. It shows the your when the iPhone was built (9=2009, 0=2010). If it's NOT 9, you have new bootrom.

If the 3-rd digit is 9, you have a chance. Now look for 4-th and 5-th digits, they show the week number. First iPhone 3GS models with new bootrom were manufactured on 40-th week of 2009. After 45-th week all models had new bootrom. Before 40-th week - old bootrom. Between 40 and 45 - cannot say for sure, you might have either old or new bootrom, just use method 2.

So:

1. Look for your (3-rd digit): if it's 9 - continue, if not 9 - new bootrom.
2. Look for week (4 and 5 digits): if 39 and below (f.e. xx939xxxxxx) - old bootrom. If 45 or higher (f.e. xx945xxxxxx) - new bootrom. If between 40 and 45 - cannot say for sure.

Examples:

1. 85938123A416 - old bootrom, because year (3-rd digit) is 9 and week is 38 (<40).
2. 85951123A416 - new bootrom, because year digit 9, but the week is 51 (>45).
3. 85038123A416 - new bootrom, because year digitis 0.
4. 85138123A416 - new bootrom, because year digit is 1.
5. 85943123A416 - cannot  say for sure, becasue year is 9 and week is 43.

Method 2

Turn off your iPhone. Download iDetector from here ^[1]. Run it. Click big "Is My Bootrom Old Or New" button.

Now you have to put your iPhone into DFU mode:

1. Hold down Power button for 3 seconds.
2. Without releasing Power button, hold down both the Home button and the Power button for 10 seconds.
3. Release the Power button and continue holding the Home button until RedSn0w detects the device (~15 seconds).

iDetector will detect your bootrom version.

Tutorials for iOS 5 jailbreak are available here ^[2].

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