I'm creating this thread for my dad. I hope i have put it in the right spot.

He has the same camera a me (nikon d90). He however has a sigma lens (18mm-200mm) while i have a nikkor lens.

He has been noticing recently that when he goes to use the camera it wont take photo's. Well today after i looked up the code in the user manual it says that it does not recognise the lens (comes up with not being attached, however it is definitely).
I took my lens off and put it on his camera to make sure it wasn't the camera itself. The problem is definitely the lens.
It happens sporadically though. It will work fine one minute, and then the next time you use it the code comes up.
I've also noticed that some of the features don't work when the lens is working. Like the multiply shutter mode.

Is anyone else having this problem at all with sigma lenses?

i might get him to have it replaced via the warranty, as the camera and the lens isn't even a year old.