iPhone 4 shows no service after changing the display and touch!

There is a small blue diode on the logic board to the left of the battery connector - underneath the dock connector flex.

If this blue diode is missing/broken (may have been damaged during repair) then this will lead to no signal.

That is not a diod the blue ic is an inductor.

Sometimes people ruin the antenna flex cable. Replacing the antenna will fix this.

Your network is totally depend upon the bottom dock. Take out the battery and then take out the speaker box downside one. then check the little pins attached with it . it may be 5 pins. it will contacts with internal base. press the pins inside while speaker base is going inside. it will fitted poperly . check it twice and the wire of speaker base will contact on the logic board . after putting inside. out the small pin which hold the battery jack and it will touch you phone's back to strength the wifi range . if these two only. these are only responsible for wifi and no service issue.

None of these answers worked for me. What I actually found is that if you follow the antenna wire up from the bottom, it routes through a couple of guides next to the battery connector. At the end of the antenna, there is a small clip that actually needs to snap INTO the socket on the board - similar to how all of the other connectors attach, but this is just a single pin. Boom - 5 bars!

I did this on a Sprint (CDMA) iPhone 4. Hope this helps some of you.

Hi has the same problem with my iphone 4 that was broken the digitizer lcd,i replaced a new digitizer lcd screen purchased from ebay.after installed ,my iphone 4 having signal problem,it say NO SERVICE/SEARCHING,then i took the motherboard and installed in to another iphone 4 that from my son ,and it work find.so i have to purchased another lcd screen from ebay again,but from other seller,and this time work for me.so this is let all you guy know that worked for me.hope this help.good luck you guys.