I actually bought a stem for an S250 from the dealer, installed it a year or so later, it didn't work. The grease they assembled it with was sticky and stopped the internals moving. I also bought an S150 from auction that must have been sitting for a few months, one of the stems was stuck closed.
I found that removing the stem and putting the coil on it and operating the aux hydraulics, you can generally feel the stem buzz in your hands. If you can't feel anything, time for new stems or try pulling them apart.
I had nothing to loose, i pulled the stems apart and cleaned them, they work great now. Better than throwing a few hundred $ at the dealer for another new stem.