Hi
From what I know iTunes error 40 is a issue related to ether the iPhone or the computer through a hardware issue
Have you tried the following :
Make sure iTunes is up to date
Reinstalling iTunes
Using recovery mode on the iPhone
If it were the iPhone I wouldn’t suspect it to be the NAND (NAND rarely fails without reason)but it could be battery related due to the phones age,old battery’s can cause all sorts of booting issues so if the above doesn’t work possibly give it a shot
If you decide to do the replacement here’s the guide you will need
[guide|19239|new_window=true]
Hopefully this helps
Any questions please ask
Thanks:-)