Direct answer: If a faucet leaks only when the water is on, the leak is usually from a worn faucet cartridge, a loose or failed faucet aerator, a bad seal at the faucet base, or a split pull-down faucet sprayer hose. The key is to find the first place water shows up, not where it finally drips.
Most likely: Most often, water showing at the spout or around the handle while the faucet is running points to a worn faucet cartridge or a loose aerator connection. Water under the sink during use usually points to the pull-down faucet sprayer hose or another faucet-side connection.
Run the faucet for a minute with a dry paper towel in hand and good light on the faucet body and under the sink. Reality check: water often travels along the faucet and drips somewhere else, so the drip location can fool you. Common wrong move: tightening random nuts hard before you know where the leak starts.
Don’t start with: Don't start by replacing the whole faucet. A lot of these are one-part repairs once you pin down the first wet point.
The leak seems to come from the end of the spout, around the aerator, or it sprays sideways instead of flowing cleanly.
Start here: Start with the aerator and spout tip. Mineral buildup or a loose faucet aerator is common and easy to confirm.
The handle area gets wet when you open the faucet, then dries back out after shutoff.
Start here: Start with the faucet cartridge branch. A worn cartridge or cartridge seal is the usual cause.
The countertop or sink deck gets wet around the faucet body while the faucet runs.
Start here: First make sure the water is not running back from the sink or splashing from the aerator. If the first wet point is under the base trim, suspect faucet base seals or a pull-down hose leak tracking upward.
The cabinet stays dry until someone runs the faucet, then you see drips from the faucet shank area or hose.
Start here: Watch the pull-down faucet sprayer hose and faucet-side connections while someone else runs the water. A split hose or leaking hose connection is common.
A cartridge leak often shows up around the handle, under the handle trim, or sometimes inside the faucet body only while water pressure is on.
Quick check: Dry the handle area completely, run hot and cold separately, and look for fresh water appearing from under the handle or trim.
If the stream sprays sideways or water beads around the spout tip, the aerator may be loose, cross-threaded, or packed with mineral debris.
Quick check: Wrap the aerator with a cloth, snug it gently by hand, then run the faucet and watch whether the leak at the tip stops.
Water around the faucet base during use can come from worn O-rings or seals where the spout or faucet body meets the deck.
Quick check: Dry the base, run the faucet without splashing the sink, and see whether water wells up from under the base trim itself.
A pull-down hose can leak only under pressure, sending water into the cabinet or up inside the faucet body where it later shows at the base.
Quick check: Pull the sprayer out, inspect the hose for a slit or rubbed spot, and watch under the sink while the faucet runs.
Most faucet leaks look like they come from the wrong place because water follows the faucet body, hose, or sink deck before dripping.
Next move: Once you know the first wet point, the repair path gets much narrower and you can stop guessing. If everything seems wet at once, reduce the flow to a small steady stream and repeat. A slower flow often makes the source easier to see.
What to conclude: A leak at the spout tip points you one way, a handle leak another, and an under-sink leak another.
A bad-looking faucet leak at the front is often just a loose aerator, mineral buildup, or water deflecting back toward the base.
Next move: If the leak stops and the stream is clean, the problem was at the faucet aerator, not deeper in the faucet. If the spout tip is fine but water appears around the handle or base, the aerator was not the main issue.
What to conclude: A loose or clogged faucet aerator is a common easy fix, but it will not cause a true under-sink hose leak.
When water shows up around the handle only while the faucet is on, the faucet cartridge is the most likely failed part.
Next move: If you clearly see water coming from the handle area during use, you have a strong cartridge diagnosis. If the handle stays dry, check the faucet base and pull-down hose next.
Water around the base can come from failed base seals, but on pull-down faucets it can also come from a leaking sprayer hose inside the faucet body.
Next move: You can now separate a top-side seal problem from an under-sink hose problem, which keeps you from buying the wrong part. If you still cannot tell, run the faucet with the sprayer head pulled out and aimed into the sink. That often exposes a hose leak more clearly.
Once the source is confirmed, the fix is usually straightforward and much cheaper than replacing the whole faucet.
A good result: A successful repair leaves every area dry during a full-flow hot and cold test.
If not: If the same area still leaks after the correct repair, the faucet body may be cracked or the fitment may be wrong. At that point, stop buying parts and consider a plumber or a planned faucet replacement.
What to conclude: Most running-water faucet leaks come down to one failed part, but repeated leakage after the right repair usually means the faucet assembly itself is worn out.
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That usually means a pressure-side leak. Common causes are a worn faucet cartridge, a loose or clogged faucet aerator, failed base seals, or a split pull-down faucet sprayer hose. The best clue is the first place that gets wet when the faucet runs.
Not always. If water appears around the base only because the stream is splashing back or a pull-down hose is leaking inside the faucet body, the fix is different. If the first wet point is truly under the base trim itself, then the base-seal path is the right one.
Usually no. A loose handle can feel sloppy, but a running-water leak at the handle is more often a bad faucet cartridge. Overtightening the handle or trim rarely fixes it and can crack parts or strip screws.
Not first. If the leak is clearly from the aerator, cartridge, or pull-down hose, those are often practical repairs. Whole faucet replacement makes more sense when the body is cracked, service parts are unavailable, or the faucet is badly worn in multiple places.
Be more cautious. A freeze can damage the faucet, but it can also split nearby supply piping inside the wall or cabinet. If you see new leaks beyond the faucet itself or the leak pattern changed suddenly after freezing weather, treat it as a broader plumbing problem.