Feb 22, 2015, 17:02
Could be anything then from mechanical issues with the focusing mechanism, dirt in the MF/AF button, that switch failure, (seems there is a little screw that gets loose on the switch)contacts to the camera... 
Sounds like a trip to the menders! Is yours the newer one with the HSM motor? Seems they are prone to fail... it's a motor and gearing assembly replacement from Sigma in that case... Is it not under warranty still? You could contact Sigma and say that the issue is known and can they not take a look at yours and give you a favourable estimate? The annoying thing is the time it's away...
Good luck and let us know how it goes? I don't use Sigma but there may be others here that would benefit from knowing...
Kind regards
Rolf
Sounds like a trip to the menders! Is yours the newer one with the HSM motor? Seems they are prone to fail... it's a motor and gearing assembly replacement from Sigma in that case... Is it not under warranty still? You could contact Sigma and say that the issue is known and can they not take a look at yours and give you a favourable estimate? The annoying thing is the time it's away...Good luck and let us know how it goes? I don't use Sigma but there may be others here that would benefit from knowing...
Kind regards
Rolf
In photography, the smallest thing can be a great subject. The little human detail can become a leitmotiv.
—Henri Cartier-Bresson