Power Steering Pump problems of the 2008 Mercedes Benz C300

I was driving to work and I felt the steering wheel was having weir problem when I was trying to turn, after I turned the steering wheel while I was driving and it didn't turn back to its own position, so I had to turn it by myself. At first, I thought it was the power steering pump or the rack had problem, but then I bought to the shop and the mechanic told me the pump and the rack worked fine and they did run computerize on my car and couldn't see any problem, but when he drove he did felt something is weir from the steering wheel stiffnes and still now we all don't know what the problem. The mechanic told me my car is still very new, it has been 6 years, it has only 39,654 and it shouldn't have any problem yet at this time.