If the serial number is XX931XXX that means it was produced in 2009 in week 31. Thus, it’s safe to flash it with the iPad baseband. If you’re already encounted with the errors above, consider having your logic board fixed
They replaced one of the components inside! As it turns out any 3GS produced after week 28 of year 2011 (sometime in July) is made with Toshiba baseband chip, as opposed to Infineon baseband chip.
If you attempt to flash it using iPad baseband 6.15.00 there is a risk of bricking your phone! Take this as a warning!
The typical symptoms of a corrupted baseband chip are:
No WiFi
No ECID
No IMEI
No Bluetooth
Please take note of serial number of up to XXX109XXX may NOT be compatible with the iPad baseband. See example below:
XX109XX = 1 is the year of 2011 and 09 is week 9 of 1 year