Canon EF 85mm f1.8 auto focus issue

[kanon]

Hi, my EF 85mm f1.8 isn't able to auto focus – it is just jumping back and forth nervously few times a second. Manual focus works ok. Has anyone had the same issue and what could be the problem?

[Rebecca32]

It might be getting stiff to the point where the motor can no longer exert enough force. This certainly afflicts the 50/1.4; the 85 is better made but my one still doesn't focus as smoothly as it once did. Does it feel stiff when manually focusing? You could vigorously move the focus ring back and forth a few dozen times to loosen it up

[kanon]

Thanks for the suggestion. It doesn't seem especially stiff and also the motor is able to focus quickly – but it's stepwise so the focus is kind of jumping back and forth.