Why a Single Faucet Has Low Water Pressure

When one faucet trickles while every other tap in the house runs fine, the problem is almost never the house. A whole-home pressure drop affects everything at once: showers, the washing machine, the other sinks. Weak flow at one fixture points to a restriction in the short path that serves just that faucet, the last few feet between the shutoff valves under the sink and the spout. That short path has only a handful of things that can clog or close, and you can check each one without tools and without shutting off your water main. This guide walks through that one-fixture lane in the order that finds the cause fastest.

First, Confirm It’s Only This One Faucet

Before chasing a fix, prove the problem is local. The fastest test is to compare the weak faucet against the nearest working one and to check its hot and cold sides separately. That single comparison tells you whether you are dealing with a clogged part at this fixture or a larger problem that only seems to show up here.

Run the troubled faucet on full cold, then on full hot, and watch each stream on its own. Then go to the next faucet over and do the same. You are looking for three patterns. If both hot and cold are weak at the problem faucet but the neighboring faucet is strong, the restriction is local to this fixture, and the rest of this guide applies. If the neighboring faucet is also weak, or if every fixture in the house is down, the cause is upstream and house-wide, not a single-faucet issue. For that case, see our guide on why your whole house has low water pressure (post 117), which covers the pressure-reducing valve, the main line, and the municipal supply. Do not try to diagnose the main or the meter from one sink.

There is a third pattern worth noting now, because it shortcuts the rest of the search: if only the hot side is weak, or only the cold side is weak, you can skip past the parts that affect both. That clue is covered in its own section near the end. A faucet that is weak on both hot and cold has a restriction after the two streams meet, which means the aerator or the cartridge. A faucet weak on only one side has a restriction before they meet, which means that side’s stop valve or supply line.

The Aerator: The Most Common Single-Faucet Culprit

For a single faucet that lost pressure on both hot and cold, the aerator is the first suspect and the most likely answer. The aerator is the small screw-on screen at the very tip of the spout, and it is the narrowest point water passes through before it reaches you. Because its openings are tiny, it catches debris that flows right past everything else.

Three things collect there. Mineral scale builds up from hard water, slowly coating and shrinking the openings. Sediment and rust flakes travel down from older galvanized pipes or a water heater and lodge in the screen. And after any plumbing work nearby, loose metal shavings or pieces of pipe-joint material wash downstream and stop at the aerator. Any of these chokes the flow or makes the stream sputter, spray sideways, or split.

You can confirm the aerator quickly. Unscrew it from the spout tip by hand or with a cloth-wrapped pliers, then turn the faucet on with the aerator removed. If the water now blasts out strong and full, you found your restriction, and the parts you removed need cleaning or replacing rather than the faucet itself. If the flow is still weak with the aerator off, the aerator is not your problem and the restriction is further back, at the valves or the supply line. The cleaning, descaling, and reassembly steps, including the trick for hidden recessed aerators, are a job of their own, covered in our guide on how to clean or replace a faucet aerator (post 024). Here, the aerator is a diagnosis: prove it or rule it out, then move on.

Checking the Supply Stop Valves Under the Sink

If both sides are weak and pulling the aerator did not restore full flow, look under the sink at the two shutoff valves, also called stops. Each fixture has its own pair, one for hot and one for cold, and a stop that is partly closed or failing will throttle the water to that faucet alone while leaving the rest of the house untouched.

Start with the obvious. A stop that was bumped, or never fully reopened after a past repair, can sit half-closed. Trace each supply line from the wall up to the faucet, find the oval or round handle on each valve, and make sure both are turned fully counterclockwise to the open position. On a quarter-turn stop, the lever should be in line with the pipe when open; if it is crossways, it is closed or partly closed. Open both fully and recheck the flow.

Valves themselves also fail in ways that restrict flow. Older multi-turn stops can corrode internally, and the rubber washer inside can swell, harden, or break loose, partly blocking the passage even when the handle reads open. Turning a stiff old valve off and back on can shake mineral chips and rust loose, and that debris travels downstream to lodge in the line or the aerator. If a stop will not turn, feels gritty, weeps around the stem, or does not improve the flow when fully opened, it has likely failed internally. Replacing a corroded or failed stop valve, which means shutting off the water upstream and swapping the valve, is straightforward plumbing but beyond this diagnosis. If you are not comfortable working with the shutoffs, a licensed plumber can replace a stop quickly.

Kinked, Corroded, or Sediment-Clogged Supply Lines

When the aerator is clear and both stops are fully open but flow is still weak, the restriction is likely in the flexible supply line that runs from each stop valve up to the faucet. These lines are short, and they fail in three predictable ways, all of which you can spot by looking and feeling along their length.

A kink is the simplest. Braided and plastic supply lines bend easily during installation or when a cabinet gets crowded, and a sharp bend collapses the inner tube and chokes the flow even though the outside looks fine. Run your fingers along each line and straighten any tight bend, then recheck.

Age accounts for the other two. The braided stainless lines common today have a rubber or plastic inner liner, and that liner degrades over years of use. Pieces of a deteriorating liner can flake off and collect at the inlet where the line meets the faucet, slowly strangling the flow. Older metal supply lines and connectors can also corrode internally, especially in hard-water areas, narrowing the passage. Both of these tend to affect just one side, hot or cold, depending on which line is aging. Disconnecting and inspecting a supply line, or replacing one, is a contained job, but it requires shutting off the stops and catching the water in the line, so treat it as the point where casual diagnosis ends. Trade guidance commonly suggests replacing flexible supply lines every several years as a matter of maintenance, since a degrading liner that restricts flow today can also fail and leak later.

When the Cartridge or Valve Itself Restricts Flow

If both hot and cold are weak, the aerator is clear, the stops are open, and the supply lines look good, the restriction has moved inside the faucet body, into the cartridge or valve. This is the least common cause on the list, which is why it comes last, but it is real, especially on a faucet that lost pressure gradually rather than suddenly.

Inside a single-handle faucet, a cartridge controls both volume and temperature, and inside a two-handle faucet each side has its own smaller cartridge or valve stem. Sediment and mineral scale can build up inside the cartridge over time, partially blocking the narrow channels the water flows through, and a cartridge can also wear or distort enough to restrict flow. The tell is that the faucet has lost pressure everywhere it could and you have already cleared the aerator, the stops, and the lines, leaving the inside of the valve as the only remaining choke point. For how the different valve types are built, see our guide on how a faucet works (post 020).

Confirming a clogged cartridge means opening the faucet, and replacing one is a defined procedure with model-specific steps, covered in our guide on how to replace a faucet cartridge (post 025). The diagnostic point here is one of elimination: the cartridge is the answer when every easier-to-check restriction in front of it has been ruled out. Reaching this conclusion by skipping the aerator and stops usually means replacing a cartridge that was never the problem.

Hot-Side-Only vs Cold-Side-Only Low Pressure

A faucet that is weak on only one temperature narrows the search dramatically, because it points straight at the supply path for that one side. Anything that affects both hot and cold sits after the two streams combine; anything that affects only one sits on that side’s own line, before they meet.

If only the hot water is weak, suspect the hot side’s stop valve and hot supply line first. A partly closed hot stop, a kinked hot line, or a corroded hot connector explains weak hot flow while cold runs fine. Hot-side restrictions also have an extra source: scale and sediment build up faster on the hot side because heat accelerates mineral deposits, and that debris can settle in the hot supply line or in the water heater’s own connections. A house-wide hot-water flow problem points back toward the water heater rather than this faucet, which is a different question covered elsewhere in the heating section of this guide.

If only the cold water is weak, mirror the logic onto the cold side: check the cold stop valve and the cold supply line for the same partial closure, kink, or corrosion. In both cases, because the aerator and the cartridge usually affect both streams together, a one-side-only symptom lets you ignore them and go straight to that side’s valve and line. That is the shortcut the opening test sets up: confirm it is one faucet, note which side is weak, and let the answer tell you where to look.

Frequently Asked Questions

Why does only one faucet have low water pressure?
A drop at a single faucet means the restriction is in the short path serving that fixture, not the house. The usual causes, in order of likelihood, are a clogged aerator at the spout tip, a partly closed or failing shutoff valve under the sink, a kinked or corroded supply line, or a clogged cartridge inside the faucet. A house-wide drop, by contrast, affects every fixture at once.

How do I know if my aerator is causing the low pressure?
Unscrew the aerator from the tip of the spout and turn the faucet on with it removed. If the water flows strong and full without the aerator in place, the aerator was the restriction and needs cleaning or replacing. If the flow is still weak with the aerator off, the cause is further back, at the valves, the supply line, or the cartridge.

Can a shutoff valve under the sink reduce water pressure?
Yes. Each faucet has its own hot and cold stop valves, and one that is partly closed or has failed internally will throttle the flow to just that faucet. Make sure both valves are turned fully open. If a valve will not open all the way, feels gritty, or does not improve the flow when open, it has likely failed inside and needs replacing.

Why is only the hot water weak at my faucet?
Weak flow on one temperature only points to that side’s supply path, because anything that affects both hot and cold sits after the two streams combine. For hot-only weakness, check the hot shutoff valve and hot supply line for a partial closure, kink, or corrosion. Scale and sediment also build up faster on the hot side, since heat speeds mineral deposits.

Should I clean the aerator or replace the supply line first?
Start with the aerator, because it is the most common cause and the easiest to check. It unscrews by hand and needs no tools or shutoff. Only move on to the stop valves, supply lines, and cartridge if removing the aerator does not restore full flow. Working in that order keeps you from replacing parts that were never the problem.

This article is general information, not professional advice. Working on shutoff valves, supply lines, or the inside of a faucet is straightforward for many homeowners, but if a valve will not close, water cannot be shut off, or you are unsure, consult a licensed plumber and follow your local plumbing code.

Sources

U.S. Environmental Protection Agency, WaterSense, Bathroom Faucets (WaterSense labeled bathroom faucets use a maximum of 1.5 gpm at 60 psi and at least 0.8 gpm at 20 psi, defining low flow): https://www.epa.gov/watersense/bathroom-faucets

U.S. Department of Energy, Federal Energy Management Program, Best Management Practice #7: Faucets and Showerheads (federal standard limits faucets to 2.2 gpm at 60 psi for fixtures made after January 1, 1994): https://www.energy.gov/cmei/femp/best-management-practice-7-faucets-and-showerheads

This Old House, How To Clean a Clogged Faucet Aerator (mineral and sediment buildup at the aerator screen reduces and distorts flow; the remove-and-test method): https://www.thisoldhouse.com/plumbing/21124360/how-to-clean-clogged-faucet-aerator

Family Handyman, Slow Running Water: Unclog the Aerator (a clogged aerator is a leading cause of low flow at a single faucet; sediment and scale collect in the screen): https://www.familyhandyman.com/project/slow-running-water-unclog-the-aerator/

Oatey, Oatey 101: Stops and Supply Lines (function of fixture stop valves and braided flexible supply lines that connect the stop to the faucet): https://www.oatey.com/faqs-blog-videos-case-studies/blog/oatey-101-stops-and-supply-lines