If you have an older shower valve, you may have experienced the effect of a stripped shower valve. You turn the handle to adjust the temperature or flow rate, but nothing happens. The problem is that the ribbed end of the shower valve stem has stripped, and the handle no longer grips it. This is a fairly common problem with older valves, and it may or may not be fixable. If it isn't, you'll probably have to replace the cartridge or compression valve depending on which type of faucet you have.
Before you do that, you can employ a couple of quick fixes. You can also try another shower handle stem repair strategy that isn't as quick but may be effective. Any of these repairs will buy time if they work, but when the shower valve stem is stripped to the point at which it can't grip the handle, it's usually time to replace it.
Two Quick Fixes for a Shower Handle Stem
If you have particularly hard water, mineral deposits, or scale, can lodge in the ribs on the valve stem and turn what is supposed to be a ribbed cylinder into a smooth one. Vinegar will dissolve scale, but it needs time to work. If you can remove the valve, you can soak it overnight in distilled vinegar. That should take care of the problem. Removing the stem might not be an option. In that case, make a paste using borax and vinegar, and cake it around the exposed stem after removing the handle. Keep it moist for several hours by spraying vinegar on it. When you rinse it off, the scale should be gone.
The stem may actually be worn, but if you can still see ribs, you may be able to get the handle working by bulking up the faucet stem with plumbing tape. Wrap the tape as many times around as it takes to make the handle fit tightly. You'll probably need at least five turns. You should have a little trouble getting the handle back on. That's good. Once you do, tighten the screw as firmly as you can and give it a try. It works? Great. You probably won't have to do anything else for at least a year.
A Not-So-Quick Fix
You may be able to deepen the grooves on the stem. The fastest and best way to do this is to use a rotary tool with a very thin grinding wheel attachment. You don't have to deepen all the grooves, but you should work on at least four, and they should be evenly spaced on the stem. If you don't have a rotary tool, you can use a file, but it will take quite a bit longer and require more effort. After you've completed this rudimentary shower knob repair, it's a good idea to replace the handle. If the ribs on the stem are worn, the ones on the handle are probably worn too.
Replace the Cartridge or Compression Valve
By far, the best way to fix the shower knob is to replace the valve. Be sure to turn off the water to the bathroom before you do this. Remove the handle first. If you have a cartridge valve, you remove the cartridge either by loosening a retaining ring with locking pliers or pulling a retaining pin using needle-nose pliers. It may take some muscle to get the cartridge out. If you can't do it, you may need to rent a cartridge puller from your local plumbing supply outlet. Take the cartridge to the plumbing supply dealer for a replacement or order one online.
If you have a two-handled compression faucet, you release the cartridge by loosening and removing the retaining collar using locking pliers. Pull the valve out of the valve housing and replace it.