Inside a shower mixer valve is a cartridge that regulates water flow and temperature. It can be made of plastic or brass, and it has a series of strategically placed holes that align with the inlet holes on the valve seat when you turn the handle. While the cartridge can sometimes break or get clogged, it is more common for the rubber seals around the cartridge and water inlets to wear out. If you are getting sufficient water flow and the valve doesn't leak, but you can't regulate the temperature, it's possible that the cartridge isn't installed correctly.
Turn off the water to the shower. If there are shut-off valves installed in the pipes leading to the shower, turn them off. If there are no valves, turn off the main water valve located in the basement or outside your house. Open the shower to relieve water pressure.
Pry the cap off of the front of the handle. If it is a round or octagonal plastic one, unscrew the screw holding the handle and take the handle off. If the handle is a metal lever, look for an Allen screw under the lever. Unscrew it with an Allen wrench and take off the lever.
Unscrew or pull off the plastic limiter if the valve has a feature that allows you to preset the maximum water temperature.
Unscrew and remove the nut holding the cartridge with a pair of Channellock pliers. Some cartridges are held in place by a clip. If so, pull it straight out with needle-nose pliers. Reattach the handle and use it to pull the cartridge out of the valve seat. Be sure to note the orientation of the cartridge before you remove it.
Inspect the rubber O-rings around the cartridge and the gaskets lining the inlet holes in the valve seat for signs of wear if the shower has been leaking. Replace any that are worn. Pry the gaskets from the valve seat with a slot screwdriver if they need to be replaced.
Look for cracks on the cartridge itself and replace it if you find any. If the holes are obstructed, clean them with the jet from a garden hose. Use a wire brush to remove any mineral deposits you find. If the deposits are extensive, or there is a significant amount of corrosion, replace the cartridge.
If it was regulating temperature correctly, replace the same cartridge, or a new one, in the same orientation the original cartridge was in when you took it out. If it wasn't, turn the cartridge 180 degrees before you reinstall it. It could have been installed incorrectly in the first place, or the hot and cold supply lines may be reversed.
Screw on the retaining nut, or put the clip back in to hold the cartridge. You shouldn't have to force the clip. If the cartridge is seated correctly, it should just slide right in.
Replace the plastic limiter, if there is one, then put the handle back on and tighten the screw to hold it. If there is a cap, snap it back in place.
You may be able to dissolve mineral deposits on the cartridge by soaking it overnight in vinegar. If you can't pull the cartridge out with the handle, don't force it or you may strip the handle screw. Buy or rent a cartridge puller to remove it.
Be sure the water is off before you remove the nut or pin holding the cartridge. If it isn't, the cartridge will shoot out in a jet of water, and you may be scalded.