When one faucet in a St. Louis home runs weak while the rest of the house flows normally, the cause is almost always close to that fixture. Before you suspect the well, the city main, or the water heater, work through the simple checks below in order. Most take only a few minutes and need nothing more than a bucket, a flashlight, and a towel.

Start by comparing fixtures

Run the weak faucet, then run a bathroom sink or tub on another floor at the same time. If every fixture is weak, the problem is upstream, in the supply line, the area around the meter, or the well system. If only one faucet is weak, the problem sits in that fixture or its branch line. This single test narrows the search more than anything else.

Check the aerator and the screen

Unscrew the aerator at the tip of the spout. Rinse out grit, flakes, and small particles, then look at the mesh. Sediment, mineral scale, and bits of rubber from old washers collect there and choke the flow. Put it back, then run the water again. If the flow improves, you have found the problem. If your water carries sediment, clean the aerator every few months.

Faucets with pull-down wands or cartridge valves may also have a small screen in the base or at the inlet. Check the manufacturer's manual for its location before you take anything apart.

Look at the shutoff valves under the sink

Under the sink, each hot and cold line should run to a shutoff valve, often called an angle stop. Turn it fully open, then give it a slow quarter turn back to see whether the handle moves smoothly. Old valves sometimes stop partway open even when the handle looks right. If you see a dripping stem, corrosion, or a handle that spins without changing the flow, the valve may need replacing. A licensed plumber can handle that job if you would rather not do it yourself.

Follow the flexible supply line

The braided or plastic hose connecting the shutoff to the faucet can kink when the cabinet is packed tight. A bent or partly collapsed hose cuts flow quickly. Gently pull the cabinet contents forward, look for sharp bends, and straighten the line if you can. Feel the hose for lumps or soft spots, which can indicate internal damage.

Consider the pipe itself

Many older St. Louis homes, especially those built before copper became common for supply lines, have galvanized steel pipe. Over the decades, mineral scale and rust can narrow the inside of those pipes. When the narrowing is on the branch that feeds one fixture, that fixture may be the only one that struggles. If the weak flow developed gradually and nothing else explains it, ask a plumber to inspect the branch line with a camera or to judge whether that section should be replaced.

Rust-colored water or a slow decline over many months are common clues that scale is involved. A plumber can judge the pipe's condition. You can also check whether the water itself carries iron or sediment by using a home test kit or sending a sample to a certified laboratory, which tells you what is actually in the water instead of leaving you to guess.

Check hot and cold separately

Run only the hot side, then only the cold side. If one side is weak, follow that line back toward the water heater or the main cold supply. If both sides are weak only at that faucet, look at the cartridge or valve inside the faucet, since worn internal parts can restrict flow even when the aerator is clean.

Know when to stop and call

  • Flow is weak at every fixture in the house
  • You smell gas, see water pooling, or notice a leak you cannot stop
  • A shutoff valve will not close or open fully
  • Cleaning the aerator and checking the supply line made no difference
  • The problem began suddenly after plumbing or yard work

Write down what you tried, when you tried it, and what changed. That short list helps whoever you call in St. Louis work faster and avoids repeating your steps. Keeping faucets and their shutoffs in working order is simple maintenance that makes problems easier to catch early.