Hard to turn the steering wheel on 2008 ML 550.

I am having the same problem on the same year make and model vehicle.... I was wondering what this might run to repair.

My belt seems to be turning fine. I'm told the power steering fluid "filter" can get dirty/clogged and is the best place to start. If that doesn't do it, it "may" need a pump. Mine actually worked OK in the really cold winter weather, but it's getting worse as it warms up again...... Whatever that means.