Low water pressure in one faucet: what to check first

Low water pressure in one faucet, with the rest of the house normal, is almost always local to that faucet. Open both shutoff valves fully, then unscrew the aerator and rinse out the mineral grit. If only hot or only cold is weak, the cartridge or an inline filter screen is next.

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By San Gabriel Handyman · Updated October 30, 2026 · 9 min read

Low water pressure in one faucet is one of the most common reasons San Gabriel homeowners text us a photo, and it is also one of the most fixable. If the other taps in the house run normally, the water supply is fine and the problem sits somewhere between the shutoff valve under that sink and the tip of the spout. That is a short stretch of plumbing with only a handful of parts that can restrict flow.

This guide walks through those parts in the order we check them, explains why San Gabriel’s water tends to clog the same few spots, and says plainly where a faucet problem ends and a whole-house pressure problem begins.

First, is it the faucet or the house?

Open the faucet that bothers you and one or two others at the same time. If everything is weak, stop reading about aerators: that points to the supply side of the house, such as the main shutoff, a pressure regulator or the connection at the street, and it is a bigger plumbing job with its own free estimate. If only one faucet is weak, you have a local restriction, and the rest of this guide applies.

Moen, one of the largest faucet makers, starts its own low-flow troubleshooting the same way, with the water supply shutoff valves under the sink: check that they have not been turned off before anything else. That sounds too simple to matter, but shutoff valves sometimes end up partly closed after under-sink work, and older multi-turn valves are easy to leave half open.

A quick order of operations for one weak faucet. Each step is cheaper and quicker than the one after it.

Is the stream actually weak, or just designed that way?

Modern faucets are built to flow less than the ones they replaced. Moen says it makes its bathroom sink faucets at 1.2 gallons per minute, standard kitchen faucets at 2.0, and pull-out or pull-down kitchen faucets at about 1.5, citing national conservation rules.

That matters when you compare one faucet to another. A pull-down kitchen spout will always feel gentler than the old two-handle faucet it replaced, and a bathroom sink will feel gentler than the kitchen. A true restriction shows up as a stream that has changed over time, sputters, sprays sideways or is clearly weaker than it was last month.

A new faucet is built to flow this much, so a weak-feeling stream is not always a fault. Figures are Moen’s published design flows.

The aerator is the first suspect

The aerator is the small screened cap on the end of the spout. It mixes air into the stream and holds the flow restrictor. Moen explains that aerators accumulate debris and mineral deposits over time, which can restrict water pressure or distort the spray pattern. The fine mesh tends to be the first place scale and grit collect.

Aerators come in three broad styles: a visible cap with threads on the outside of its shell (male), a visible cap with internal threads (female), and a concealed insert that needs the small removal key that came with the faucet. Moen’s aerator guide lists the turning direction for each style. If a cap will not budge, wrap the spout in a cloth before you use pliers, and stop rather than force it.

What sits inside the tip of a typical spout. Scale clogs the mesh first because it has the smallest openings.

Why San Gabriel water clogs the same few spots

Hardness is the amount of dissolved calcium and magnesium in water. The U.S. Geological Survey says hard water is not a health concern but can be a nuisance, causing mineral buildup in plumbing, fixtures and water heaters, and that heating water makes calcium carbonate drop out as solid deposits. The same thing happens at a smaller scale at a spout: water evaporates at the tip, and the minerals stay behind as a chalky ring and as grit in the mesh.

San Gabriel has an unusual setup. The city says it is served by five water companies, though most customers are served by the County Water District. San Gabriel County Water District’s 2025 water quality report lists hardness from 17 to 200 ppm, averaging 97.4, an average that sits in the USGS “moderately hard” band, with individual results running from soft to just into very hard. If your bill comes from one of the other four companies, its own annual report gives the number for your street.

San Gabriel County Water District’s 2025 hardness range with the average marked, over the USGS bands. Other suppliers in the city publish their own figures.

Calcium and alkalinity matter too. The same district report lists calcium averaging 28.4 ppm and alkalinity averaging 125 ppm. Those are the ingredients of scale, and they are why a faucet in one part of San Gabriel can need cleaning more often than a faucet a few blocks away on a different supplier’s water.

Weak on the hot side only, or the cold side only

When only one temperature is weak, the aerator is no longer the main suspect, because both temperatures pass through it. Moen’s troubleshooting table says a temperature-specific reduction points to the cartridge, and on some bathroom models, to filter screens fitted in the hot and cold supply lines under the faucet. Those screens can be unthreaded, rinsed and put back.

Hot-side restrictions can be more common in hard-water areas for a reason that follows from the USGS point above: heating is what pushes calcium carbonate out of solution, so hot-side parts can collect scale faster than cold-side ones. If the weak side is hot, it can also be worth flushing the water heater, which we cover on our water heater flushing page.

One quick reference table

The table pulls the pieces above into one place. Start at the top row that matches what you see.

Where one weak faucet usually loses its flow
What you notice	Most likely cause	First thing to try
Weak flow, hot and cold both	Clogged aerator, or a stuck diverter or wand screen on a pull-out spout	Remove and rinse the aerator or wand screen
Weak on hot only, or cold only	Debris in the cartridge or an inline filter screen on that side	Clean the inline filters if the model has them, else replace the cartridge
Weak since you touched the plumbing	A shutoff valve under the sink not fully open	Open both shutoffs all the way
Weak at the sprayer only	Diverter stuck, or sprayer hose or wand blocked	Work the sprayer trigger a few times, then test the hose alone
Weak at every tap in the house	Supply side: main valve, regulator or the street connection	Bigger plumbing job; free estimate

Older San Gabriel homes add a few wrinkles

The Census Bureau’s American Community Survey puts San Gabriel’s median home at 1957, and roughly 38 percent of its homes were built before 1950 (Census Reporter, tables B25035 and B25034). Houses that old often still have their original shutoff valves, and those are the parts most likely to be partly closed, corroded or leaking at the stem.

If turning a shutoff does nothing, or it weeps when you move it, do not keep working it. Our shutoff valve replacement work swaps them like for like, and it is also covered in our guide to shutoff valves in older San Gabriel homes. A weak faucet with a fresh shutoff valve and a clean aerator is a very different diagnosis from one with neither.

Cleaning the aerator without hurting the finish

Rinse first. Often the grit comes straight out under running water. For scale that is stuck on, a soak of the removed aerator parts in white vinegar is the usual home method; Moen’s finish-care page also lists a 50/50 mix of household vinegar and water for water spots on the faucet itself, and says to rinse the faucet thoroughly after any cleaner and dry it with a soft cloth, and to avoid abrasive pads and harsh lime-scale removers on polished or specialty finishes.

Hansgrohe, another maker, recommends a gentle citric acid cleaner and a quick wipe-down after each use in hard-water areas. Remove the aerator parts from the spout before soaking them, and keep the vinegar off the spout body itself.

When the aerator is clean and the flow is still weak

On a pull-out or pull-down kitchen faucet, Moen’s next step is the screen between the spray wand and the hose. Unthread the wand, hold the hose over the sink and run the water. Strong flow from the open hose means the wand screen is the problem. Weak flow from the open hose points back to the cartridge, which is replaced and the valve body flushed.

Two-handle faucets have a cartridge or stem on each side, so a weak hot side means the hot-side part. If you have to replace the part, bring the old one or send us a clear photo with the brand, because cartridges are brand and model specific. Our dripping faucet repair page covers rebuilds, and a faucet that is worn out or no longer has parts available can be swapped through faucet installation.

What we do on a visit, and what we hand off

For a single weak faucet we check the shutoffs, clean or replace the aerator, test each temperature, and replace the cartridge or supply line if needed. We can also replace the shutoff valves while we are under the sink.

When every faucet is weak, or the pressure swings from tap to tap, the question is no longer a faucet question. A pressure regulator that is failing is something we can look at through our pressure regulator and expansion tank work, and anything involving the supply lines in the walls or the water main is a bigger plumbing job with its own free estimate. We would rather tell you that on the first text than spend a visit on the wrong part.

Related reading: dripping faucets in older San Gabriel bathrooms covers the worn-washer problem that often turns up alongside scale, and our guide to stiff faucet handles covers the cartridge side in more detail.

Frequently asked questions

What causes low water pressure in only one faucet?

A local restriction between the shutoff valve and the spout tip. The usual suspects are a partly closed shutoff valve, a clogged aerator, debris in the cartridge or an inline filter screen, and on pull-out faucets a blocked wand screen or stuck diverter.

How do I fix low water pressure in one faucet?

Open both shutoff valves fully, unscrew the aerator and rinse out the grit, then test again. If only hot or only cold is weak, clean the inline filter screens if your model has them or replace the cartridge for that side.

Why is the pressure low on just the hot side of one faucet?

Both hot and cold pass through the aerator, so a one-temperature problem points to the cartridge or a supply-line screen. Heating water makes calcium carbonate drop out of solution, which is one reason hot-side parts can collect scale faster in hard-water areas.

Can hard water cause low pressure at one faucet?

Yes. The USGS notes hard water can cause mineral buildup in plumbing and fixtures, and Moen notes that mineral deposits in an aerator can restrict water pressure. It is a nuisance rather than a health concern.

Is it safe to clean an aerator with vinegar?

For the aerator parts themselves, a vinegar and water soak is a common method. Keep the vinegar off polished or specialty finishes on the spout body, rinse everything thoroughly and dry with a soft cloth, as Moen advises.

When should I call someone instead of fixing it myself?

Call when a shutoff valve will not turn or weeps, when a cartridge will not come out, or when more than one faucet in the house is weak. Text a photo of the faucet and the area under the sink and we can usually tell which kind of job it is.

Sources

Ready to hand it off? Our plumbing fixes page lists what we take on and what to send for a quote. For background on local houses, see our repair guide for San Gabriel’s older homes.

Want this done for you?

Send a photo to (310) 628-9399. We work Mon–Sat 8:30 AM–7 PM, and the estimate is free before anything starts.

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