Repairing a bathtub faucet isn't the difficult job that it sounds. The main piece of equipment inside a faucet is called the valve stem. This is the part that is attached to the faucet handle. As you rotate the handle, it turns the stem, which raises and lowers a rubber washer that controls the flow of water through the faucet. These valve stems can sometimes become encrusted with calcium or other sediment, which causes them to stop working. The rubber washers at the end, likewise, can sometimes tear or rip. Repairing a bathtub faucet yourself is also a way to save money instead of using a plumber.
Cut off the water supply for the bathtub where you are repairing the faucet. You may have a water supply cut-off for the tub in your bathroom somewhere, perhaps behind an access panel. If you have an older house, look for the water utility box (usually made from concrete) buried in your front yard near the street. Open the cover of this to find the main water shutoff valve for your house. Rotate this valve in the opposite direction it is currently in to shut off the water.
Remove the screw cap cover on the bathtub faucet. The cap is usually disguised as the "Hot" and "Cold" labels. Use a flat-bladed screwdriver to pry the cap off, then remove the faucet screw with a Phillips screwdriver. Pull the faucet handle off the valve stem by pulling it straight out.
Look closely at the valve stem. You will see two hexagonal nuts. Remove the first nut, the packing nut, by rotating it counterclockwise with the wrench. Pull the nut off the stem. Remove the second hexagonal nut, which is the valve stem, by turning it counterclockwise as well. Pull the valve stem out of the socket.
Examine the valve stem. Use a wire brush to scrape away any calcium deposits or other sediment. Turn the valve over and look at the rubber washer located on the bottom. If it is torn or cracked, it can be repaired by removing the washer screw with a screwdriver. Replace the washer, insert the screw and tighten.
Apply pipe thread compound to the threads on the valve stem. This can repair many bathtub faucet problems, especially leaks. Tighten the stem in the socket with the wrench. Squeeze a few drops of silicone grease on the long portion of the valve stem. This will help lubricate the faucet. Slip the packing nut over the stem and tighten.
Replace the faucet handle. Insert the faucet screw and tighten. Press the faucet cap back into place.
Restore the water supply. Open the faucet in the tub. Check for proper operation.