iPhone 3GS running iOS 4 GM or iPhone OS 3.1.2 (mine was running a jailbroken copy of iOS ... Now, Pwnagetool will ask you if your iPhone has ever been jailbroken and will try to put it in DFU mode. I ...

If performed correctly, the iPhone 16’s display will remain black, indicating DFU mode is active. Quick-press the Volume up button and release. Quick-press the Volume down button and release.