Adjustable office chairs are designed to make the long hours of office work more comfortable. However, if the chair fails to function properly it becomes more of a nuisance than a convenience. A faulty gas cylinder is the likely cause of seat that doesn't move, that rises when it is not in use or that moves up and down as you are sitting. If the seat tips, flops or rocks back and forth, the seat mechanism needs to be replaced.
Lay the chair upside down. Remove the base of the chair. The base is secured to the chair with retaining clips or with washers and bolts. Pry the retaining clips loose with a screwdriver or needle nose pliers. Use an adjustable wrench to loosen and remove the nuts and bolts.
Unscrew the screws on the seat mechanism with a Phillips head screwdriver. The seat mechanism is the black metal plate directly underneath the seat. Pull the plastic skirt down to expose the chair cylinder. The seat mechanism and chair cylinder are connected with a pressure seal.
Tap the four corners of the seat mechanism with a rubber mallet while pulling down on the cylinder. The vibrations will break the seal between the cylinder and the seat mechanism. Remove the old seat mechanism.
Install the new seat mechanism on the chair according to the manufacturer's instructions. Replace the base on the chair. Secure the base with the retaining clips, screws or with the washers and bolts. The retaining clips simply clip in place. Use an adjustable wrench to tighten the washers and bolts. Set the chair upright and sit in it to test it. Rotate the seat several times to make sure that is secure and not floppy or tilting.
Lay the chair upside down. Remove the base of the chair. The base is secured to the chair with retaining clips or washers and bolts. Pry the retaining clips loose with a screwdriver or needle nose pliers. Use an adjustable wrench to loosen and remove the washers and bolts.
Twist the gas cylinder with a pipe wrench. The gas cylinder is located in the column in the back of the chair. It is a black tubelike object that is 3/4 to 1 inch in length and approximately 1 inch in diameter. One end is tapered. If the gas cylinder won't move spray it with WD-40. Twist and pull the pipe wrench until the gas cylinder pops from the column.
Place the tapered end of the new gas cylinder into the column. Twist and rotate the cylinder until it pops into place. Reassemble the base and set the chair in the upright position. Lower and raise the height of the chair several times to make sure it functions properly. Check the chair after a couple of hours to make sure that seat has not moved up or down.
Measure the gas cylinder. Purchase a new cylinder that is the exact size.
Wear eye protection when removing the gas cylinder. The cylinders contain nitrogen. If the cylinder is punctured it will release a small amount of nitrogen.