If the seat of your Herman Miller Aeron Chair won't go up and down anymore, and it's stuck in it's lowest setting, the height adjustment is broken. Replacing the cylinder will likely fix the problem. Replacing it is easy and inexpensive, and you can save money by doing it yourself.
Make sure the cable is not broken. Remove the cover at the base of the chair and push the lever. If you see the cable moving the part under the base, then the cable is not broken and replacing the cylinder will likely fix the chair's height adjustment problem.
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Look at the sticker under the seat to determine whether you need a 1-stage or a 2-stage replacement cylinder. The second number, probably in parentheses, will tell you which cylinder the chair needs. For example, if you see the number AE1(2)3, you need to buy a 2-state cylinder. AE1(1)3 means you need a 1-stage cylinder. Do not use a 2-stage cylinder to replace a 1stage cylinder -- it will not fit.
Find a replacement cylinder at a decent price on one of the online auction site or locate an online chair parts store that has the part. You can get a replacement directly from the Herman Miller company, although you will probably pay a premium price there.
Turn the chair over, use a screwdriver or pliers to remove the retaining clip at the center of the star base and remove it.
Twist the base of the old cylinder with a pipe wrench and remove it. It will require significant force to break the old cylinder free.
Press the new cylinder into the slot where the old one was. Use a rubber mallet to secure the cylinder to the bottom of the chair.
Reattach the star base. If it requires a lock clip, remove the black outer cover of the cylinder.