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If the faucet has a bad flow of water coming out, the aerator (Image 1) that is screwed onto the end of the faucet may need replacing. If water is dripping outside of the stem, the unit may need to be rebuilt. It's a good idea to rebuild both sides of the faucet if one side goes bad.
Turn off the water supply to both the hot and cold lines under the sink (Image 2).
Remove any caps from the handles being sure not to damage them. Remove the handle screw using a Phillips head screwdriver (Image 3), and then remove the handles.
Loosen the compression nuts using a pair of slip joint pliers, and then remove.
Using a pair of pliers -- grab the old cartridges, and then gently rotate them until they slip out of the faucet body.
Use a screwdriver to remove the washers and springs found under each of the cartridge bodies.
Determine why the faucet is leaking by checking the O-rings, stems, seals and springs.
A kit may be purchased for each type of faucet so everything can be replaced at once, or the part may be purchased individually for replacement.
Pay attention when installing the stems to make sure they are in the correct position for the handles.
Use a small screwdriver as a guide when installing the springs and seals.
After replacing both sets of springs and washers, place the cartridge in the cold side of the faucet body according the manufacturer's instructions, and then screw on the compression nut and tighten.
Repeat the previous step for the hot side. Make sure the cartridge is installed in the proper position and the compression nut is tight.
Replace the handles as before, and then replace the caps. Install the new aerator by simply screwing the aerator to the faucets spout.
Turn the water back on, and then check under the sink for leaks.