front passenger door lock immediately pops back up
 

When locking the doors using any method (remotes, either armrest, key on driver's side), all doors do indeed lock, but the front passenger door lock immediately pops back up again - down-up, down-up each time I press "Lock". If I press Lock over and over again, it sometimes will eventually stay locked, but most often, I have to depress the lock manually. Just in case it would help, I checked the related fuses and they seemed fine.

Do I need a new door lock? Can I order it on ebay and replace it by myself? How? thanks a millions

My S60 did the same thing and then one day it fixed itself. Don't know what was wrong and what changed.

I had one that wouldn't go down all the way and I removed the insert that goes around the locking knob and reinserted it. It seems the ring had somehow gotten a tiny bit crooked and was stopping the knob from going all the way down. Could be something similar. Hope it helps.:)