Why is my Ethernet input spring not working?

Hi @allan16030

Without actually seeing it, it seems as though the RJ45 LAN socket is damaged somehow.

It it has then it will have to be replaced.

Unfortunately it is hard mounted on the motherboard, so this means that the motherboard will have to be removed from the laptop so that the existing RJ45 socket can be unsoldered from the board and a new one soldered in its place.

Here's the hardware maintenance manual for the laptop series.

Go to p.63 (p.67of 121 pdf) in the manual to view the procedure to remove the systemboard from the laptop case.

Search online for rj45 socket for lenovo z400 laptop to find suppliers that suit you best.

If doing this yourself seems too daunting, contact a reputable, professional laptop repair service and ask for a quote to replace the socket. You may save some of the cost of the repair, by taking just the motherboard and also the RJ45 socket so that all they have to do is the soldering as you will do the removal and the replacement of the systemboard