S40 central locking won't lock doors

Hi all,

I'm having an issue with my S40. When i use the remote control to try
to lock the doors, it makes a sound and the lights flash, but the
buttons won't go down. I've got a feeling that the motor in the
drivers door may have warn out. It has been making a straining sound
for a while. I assume that because it's 'sticking' the system refuses
to lock any of the doors. I can lock/unlock the doors with the key.

Just wondering if anyone has had this problem / can verify my
diagnosis? If I do need to replace the motor, i'm curious to know if
that's a relatively 'generic' part or something that's volvo
specific. Also curious to know if it's easy to replace.

Thanks in advance!