Updated your factory unlocked iPhone 3GS to iOS 6 firmware and found out an iTunes ‘No SIM Card Installed’ error. You get this error because iOS 6 firmware updated your factory unlocked iPhone 3GS to 5.16.07 baseband and Apple restricted this baseband and you are getting this ‘No SIM Card Installed’ error.
Here is quick fix for this error you just need to downgrade you iPhone 3GS baseband to 5.13.04 which is factory unlocked on iOS 5.1.1 firmware by using redsn0w 0.9.14b2 which can downgrade iPad baseband to 5.13.04
Things you need to download:
1) Open up Redsn0w. Go to Extras > Select IPSW button in redsn0w to point to the original firmware you just downloaded.
2) Do a controlled shutdown of your iPhone (“slide to power off”).
3) On Redsn0w screen click ‘Jailbreak’. Check the ‘Downgrade from iPad baseband’ checkbox, uncheck Cydia and click Next.
4) Redsn0w will start doing the downgrade and you will see ‘Flashing Baseband’ screen on your iPhone with the Pawnapple icon. DO NOT INTERRUPT your iPhone while baseband flashing is in progress.
5) Once done, your iPhone will reboot and the baseband should have been downgraded to 5.13.04. Try to connect to iTunes once more and see if it would activate.