Your single handle Kohler shower diverter handle won't turn, the faucet is leaking, there is no hot water in the shower, or there is no water flowing through the showerhead. All of these problems are caused by a faulty cartridge and valve. Problems with the faucet caused by the cartridge usually stem from worn rubber parts on the cartridge and valve. To fix any one of these problems, take the faucet apart, remove the valve assembly and cartridge and install new replacement parts.
Locate the main water supply valve near your water meter and turn off the water to the house. Turn on the faucet in the shower to discharge line pressure and drain water from the lines. Lay towels across the drain and the entire shower floor to protect it and to keep from losing parts down the drain.
Remove the diverter handle. Locate the screw under the handle and use an Allen wrench to remove the screw then lift off the handle. Older metal and acrylic handles hide this screw under a decorative button. Use a screwdriver to lift the button off the faucet, or use pliers to loosen a threaded button, and then unscrew it by hand to remove it. Use a screwdriver to remove the handle screw and lift the handle up and off. Use a handle puller to remove immovable faucet handles. Pull the sleeve, handle adaptor, insert or washers (on certain models) off the faucet.
Remove the escutcheon (wall plate). Remove the screws from the front of the escutcheon using a screwdriver, then lift the escutcheon from the wall. On certain models, the escutcheon is held on by a threaded sleeve. Unscrew the threaded sleeve counterclockwise by hand then remove it and the escutcheon.
Remove the screws from the gasket or flange retaining the valve assembly using a screwdriver. Pull the valve assembly straight out. Be prepared with a towel to clean up water that will rush out of the faucet as you remove the valve. Pull the cartridge (look for a part with two chambers) out of the faucet. Use a toothbrush or wire brush to clean away corrosion from the faucet.
Push the replacement cartridge into the faucet. Turn the replacement valve so the mark signifying "top" is up. Check the manufacturer's instructions for the correct valve position. Push the replacement valve into the faucet and line up the screw holes on the gasket with the holes in the faucet. Replace the screws and tighten them with a screwdriver. Reverse your steps to reassemble the faucet. Remove the towels from the shower floor. Turn on the main water valve and test the faucet.