Headphone Jack Not Recognized

[JimSmith4321]

My iMac (Intel Core 2 Duo, about two years old) does not recognize any headphone jack inserted into the headphone port. That is, the Sound preferences panel lists Internal Speakers only. All jacks I've tried measure 3.5 mm diameter.
Any hope of fixing this at home or is a visit to an Apple service center in my future?

 

[SYQUEST]

It could be a software issue. I remember something like that on MACFIXIT during one of Apples buggy updates. You may have to do some searching to find it.

....JIM....