This article provides an exhaustive guide on what a flash file is, where to find the correct version for your i800, how to flash it safely using the right tools, and troubleshooting common errors.
If you see "Invalid IMEI" or "IMEI Null," the device cannot register to any cellular network. Flashing the NVRAM partition from a valid flash file can restore the IMEI (though you may need to write back your original IMEI using a tool like Maui META).