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I recently gave my Sigma 85mm DG EX HSM for repair to a local camera repair shop to fix the MF ring which did not feel "normal" after an old lens drop. When I got it back, the MF was indeed easier but the AF stopped working! Specifically, the lens does some back and forth fast spasmodic movements and then it quits trying. It does not even come close to focusing. Behavior is similar with viewfinder and live view. My camera is Canon 6D.

Can someone give me an indication of which part might be malfunctioning? Since focus servo mechanism is controlled by the camera, I suspect a flex cable damage, am I right?

PS: Please do avoid obvious responses like "send it to Sigma", "have the shop pay the damage", etc.

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    Why not "have the shop pay the damage?". They broke it, they should fix it. – Philip Kendall Sep 21 '16 at 8:12
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Take it back to the shop where you had it adjusted and have them adjust it correctly this time. It sounds like they loosened the connection between the USM motor and the focusing element a bit too much and the HSM motor doesn't have enough friction to move the focusing mechanism.

P.S. If they broke a ribbon cable nothing would move at all when the camera tries to AF.

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