Modchip Hotswap
If you have a modchip flashed with a bad BIOS, you can perform these steps using a working modchip to recover the bricked modchip to a working state. This process involves swapping the modchip while the system is running, so avoid touching anything on the motherboard, and particularly the power supply, that you don't need to.
Performing the Hotswap
- Loosen the screw near the LPC. Desolder the d0 wire from the modchip, leaving the other end attached to the motherboard, and put it under the screw and tighten it to connect it to ground. Remove the broken modchip.
- On both the good and bad modchips, solder BT to ground so that the modchip is always active, if you hadn't already set them up that way. Insert the good modchip into your Xbox.
- With your Xbox powered off, move the disc tray so that it is out of the way of the modchip, but has clearance to insert/remove a disc.
- Power on your Xbox and boot into an installer disc capable of performing a BIOS flash. Navigate to the BIOS flashing utility but do not begin the flashing process.
- With the system still running, pull the new modchip off of the LPC header and replace it with the old modchip. Do not force it on.
- Proceed with the flashing process. Once it is finished, reboot your Xbox to verify that the chip is now working, then turn it off and solder d0 onto the old chip again.
Source: Consolemods.com