Slow or no flow from an under-sink water filter is most often the filter cartridge itself — most last 6–12 months before sediment and dissolved minerals clog them enough to noticeably restrict flow. After the cartridge, the next most common causes are a partly closed shutoff valve, a kinked supply tube, or a clogged faucet aerator that has nothing to do with the filter system at all.
Check the filter cartridge age first
This is the single most common cause of gradually worsening flow. As sediment and dissolved minerals accumulate inside the cartridge, water has to push through an increasingly restricted path — homes with harder water or higher sediment levels will clog a cartridge faster than the general 6–12 month estimate. If you cannot remember the last replacement date, this is the first thing to address regardless of other symptoms.
Confirm the shutoff valve is fully open
Under-sink filter systems typically tap into the cold water line through a dedicated shutoff or feed valve. Confirm this valve, along with the main cold water shutoff under the sink, is fully open rather than only partially — a valve that was not fully reopened after any previous maintenance is an easy thing to overlook.
Check the tubing for kinks
- Trace the tubing from the shutoff valve to the filter housing
Look for any sharp bends, especially where it was pushed into a tight cabinet space.
- Check the tubing from the filter to the faucet
The same kink risk applies on the output side, not just the supply side.
- Straighten any kinks found
A kink restricts flow the same way a cartridge clog does, and is often mistaken for a filter problem since the symptom looks identical.
Check the faucet aerator
Remove the aerator from the tip of the faucet (if the filter has its own dedicated faucet) and test the flow directly from the spout. If flow improves noticeably, the aerator itself was clogged with sediment — clean it or replace it rather than assuming the filter cartridge is at fault.
Rule out your home’s general water pressure
If cold water pressure elsewhere in the house is also noticeably low, the filter system is not the cause — the underlying low pressure is a household plumbing issue rather than something specific to the filter. Confirm pressure at another cold tap before spending more time on the filter system itself.
Replacing the cartridge correctly
After installing a new cartridge, run the water for several minutes before judging performance — new cartridges often release a small amount of carbon fines initially, and flow can feel different until the cartridge has fully wetted out. Judge flow rate after this initial run, not immediately on first use.
