If your faucet won't turn off or runs from the spout when the handle is in the off position, then a faulty stem or cartridge component is usually the culprit. The mechanism inside a faucet valve housing, called a stem or cartridge, acts as an in-line shut-off valve.
In basic terms, turning the faucet to the off position causes rubber or plastic components to compress across an opening or otherwise cut off water flow. If one of the faucet's internal components breaks or becomes worn, you must replace the component to fix your leak.
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Check the Faucet's Handles
Troubleshooting a running faucet begins with checking to ensure that the faucet's handles are in a fully closed position. In particular, homeowners often forget to fully close both sides of a dual-handled faucet assembly or one of the three handles included on a tub faucet and diverter assembly. Whereas round, knob-style handles generally rotate clockwise to the off position, the angled handles of a dual-handled assembly often turn away from the spout to achieve an off position.
Because opening up a faucet's guts may involve turning off the water to the house, especially if it's a shower or tub faucet, it's best not to do it until you verify that you're actually using the faucet correctly.
Common Causes of Running Faucets
Worn rubber washers are often the cause of a constantly running faucet. Rubber washers attach by screw to the innermost end of compression-style faucets. In the closed position, the washer compresses over an opening to halt water flow. Years of use reduce the washer's size until it no longer covers the entire opening. Worn washers cause both dripping and running faucets.
Faucets without washers, such as disc, ball and cartridge-style faucets, contain several plastic and metal components that are susceptible to wear and deterioration. They can also collect mineral deposits that prevent them from sealing. You can clean mineral deposits by soaking the faucet parts in vinegar, but first you have to disassemble the faucet to extract them. Vinegar deteriorates rubber parts, so you should always replace them.
Removing Faucet Assemblies
As Delta explains, plumbers use several tools to remove and repair faucet stem assemblies. Whereas common screwdrivers and wrenches remove compression faucet stems, removing washerless faucet stems typically requires special stem-pulling tools or cartridge removal tools. Although generic stem-pulling tools are available at most hardware stores, many proprietary cartridge removal tools must be purchased directly from manufacturers or special-ordered through plumbing supply outlets.
Replacing Faucet Stems
If a leak persists after replacing worn parts, full replacement of the faucet stem may necessary. For many do-it-yourself plumbers, the most difficult part of replacing a faucet stem is finding the proper replacement. A trip to the local home improvement store presents the novice plumber with a staggering variety of faucet stems.
The most effective way to find the proper replacement for your faucet stem is to bring the old stem with you to the store. Alternatively, manufacturers typically organize stems according to the make and model of the faucet they occupy; use your faucet manufacturer's name and model number to find a match for your replacement project.