I ran into a not workin front camera after a screen replacement on Iphone4. I found out that I had broken the tiny jumper on the main board when I pried off the headphone plug. That jumper is next to the right upper corner of the headphone/volum
e button plug (yellow marked in step 17 of relacement guide), you can see it in the picture as two silvery dots. If your prying tool sits on that jumper when levelling the plug off, it is history. I took a ampoule of conducting silver liquid, but instead of shaking it I took some of the sediment out with a thin wire and aplied a tiny blob over the two remaining soldering pads left. And alas, it worked ! Front camera is ok again. Thanks to those who found out about that jumper.