If pressure is low at one faucet but fine everywhere else, the problem is almost always local to that fixture. A quick visual check at the faucet tip and under the sink will usually tell you what’s going on, so you can describe it clearly to a licensed plumber.
The Aerator: First Thing to Inspect
The aerator is the small mesh screen screwed onto the tip of the faucet spout. It mixes air into the water stream and catches mineral deposits over time. A partially clogged aerator is the most common cause of low pressure at a single faucet, and it’s easy to spot.
Look at the tip of the spout. White or brownish buildup on the screen is calcium and mineral deposit. If it’s visibly clogged, that’s almost certainly your cause. Replacement aerators are inexpensive, and a licensed plumber can clean or swap one quickly as part of diagnosing the fixture.
On kitchen faucets with a pull-down spray head, there’s often a flow restrictor built into the aerator assembly at the spray tip, and sometimes a second one where the hose connects to the head. If flow is weak only at the spray setting, mention that to your plumber.
The Shutoff Valve Under the Sink
If the aerator looks clear, check under the sink. There’s typically a hot and a cold shutoff valve where the supply lines connect to the wall or floor. These get partially closed during repairs or move-ins, then not fully reopened.
A ball valve (lever handle) should point in line with the pipe when fully open. An oval-handled compression valve opens counterclockwise. You can check that both are open all the way. That’s a safe, no-tools inspection.
Old compression valves can fail internally. They look open but the internal parts have worn and restrict flow. If a valve leaks when you operate it, or turning it doesn’t change anything, it needs replacement. That means shutting off water to the house, which is a job for a licensed plumber.
Supply Lines: Look for Kinks and Age
The braided flexible hoses connecting the shutoff valves to the faucet are easy to inspect visually. Open the cabinet and see how they’re routed. If something got shoved into the cabinet, a line may have kinked. Straightening a kinked hose is a no-tools check.
Also look for discoloration or swelling on the hose itself. Supply lines more than 10 years old, especially original plastic-braided ones, can corrode internally. The outer braid looks fine while the inner liner has collapsed. A swollen or discolored supply line needs replacement, not cleaning.
What May Be Happening Inside the Faucet
If the aerator, valve, and supply lines all check out, the issue is likely inside the faucet body. Cartridges and ceramic disc assemblies crack and wear over time. Debris lodged in a cartridge restricts flow on one temperature only, hot or cold, which shows up as weak pressure on that side. Diagnosing and replacing a cartridge requires identifying the exact part and disassembling the faucet body.
In older homes, particularly pre-1970s construction, galvanized iron pipes corrode from the inside over decades and the internal diameter narrows slowly. If pressure is getting worse at multiple fixtures, not just this one, galvanized supply lines may be the real issue. A licensed plumber can assess whether a partial repipe makes sense.
When to Call a Licensed Plumber
If you’ve looked at the aerator, confirmed both shutoff valves are fully open, and inspected the supply lines for kinks and age, you have enough information to make a useful call. A plumber will diagnose and fix it from there.
Call one if:
- The aerator is visibly clogged
- A shutoff valve leaks when operated, or won’t open all the way
- Supply lines are swollen, discolored, or more than 10 years old
- Pressure is still poor after the aerator and valve check out
- You see any drips or active leaks under the sink
In California, plumbers must hold a C-36 license. Verify any contractor before work begins at cslb.ca.gov. Low pressure at one faucet is usually a quick, inexpensive repair for a licensed pro. Getting it diagnosed right the first time prevents a leak inside the cabinet down the road.