Reverse Osmosis Troubleshooting: No Water, Slow Faucet, Noises, and TDS Creep
A reverse osmosis system rarely fails loudly. The faucet just gets weaker, or the drain runs longer than it used to, or a TDS meter reads a little higher than last month. Most of these problems trace back to pressure, the storage tank, or a worn membrane, and you can sort through them with a pressure gauge and a TDS meter before you call anyone.
Work in order. Confirm the pressure and the tank first, since those are cheap and common, then move to the parts that cost money. This page covers diagnosis by symptom. If you are installing a new system, the under-sink RO install guide walks through the setup itself.
Start With Feed Pressure and Tank Charge
Two readings explain most RO complaints, and both take a couple of minutes.
The first is feed water pressure. An RO membrane is a pressure-driven filter. It needs incoming pressure of roughly 40 to 60 psi to push water across the membrane and leave dissolved solids behind. Below about 40 psi the flow drops off and the membrane stops rejecting contaminants as well, which the EPA notes is part of why point-of-use RO performance depends on operating within the system’s rated conditions. Put an inexpensive gauge on the cold supply line under the sink and read it. Low house pressure is the single most common reason for weak flow.
The second reading is the storage tank pre-charge. The tank holds an air bladder that pushes filtered water out to the faucet. Drain the tank fully, then check the air valve on the side with a low-pressure tire gauge. Empty pre-charge should sit around 7 to 10 psi on most under-sink tanks. A tank that reads near zero is waterlogged, and it will dribble at the faucet no matter how healthy the membrane is.
Get those two numbers before you open anything. They tell you fast whether the trouble is pressure, the tank, or the membrane itself.
No Water at All
A faucet that produces nothing is almost always a supply or tank issue, not a dead membrane. A membrane fails by leaking solids through, not by stopping flow.
Check the obvious shutoffs first. The cold feed valve has to be open, and the small ball valve on top of the storage tank has to be open too. That tank valve gets left half closed after filter changes more often than anyone admits, and a closed tank valve traps all your filtered water in the tank.
If both valves are open and still nothing comes out, look at feed pressure. Below 40 psi the system can stall completely. A booster pump, sold separately, fixes a chronic low-pressure house. After that, a sediment pre-filter clogged solid can choke off flow to the membrane entirely. If the pre-filters are months overdue, change them and see if production returns. The filter replacement guide covers the schedule for each stage.
Slow Flow or a Trickle From the Faucet
A trickle is the most common RO complaint, and it is usually the tank, not the membrane.
When the storage tank loses its air charge, water still gets into the tank but nothing pushes it back out, so you get a sad stream that fades after a cup or two. Drain the tank, check the pre-charge against that 7 to 10 psi target, and add air with a bike pump if it reads low. Add it in small bursts and recheck, since these tanks hold very little air.
If the tank charge is fine, look downstream. A clogged final post-filter, the small carbon polish stage between the tank and faucet, throttles the last bit of flow. A kinked or pinched tube does the same. And if feed pressure is low, every stage runs slow, which loops back to the first reading you took.
One note on speed in general. RO is slow by design. A standard under-sink system makes filtered water by the drop and stores it in the tank, so the faucet flow you feel is really the tank emptying, not the membrane keeping up. A weak draw that recovers after the tank refills for a few hours is normal. A weak draw that never recovers is the problem worth chasing.
Noises: Gurgling, Hissing, and a Running Drain
Sound is a useful diagnostic once you know which sounds are normal.
Gurgling at the air gap while the system is producing water is normal. The air gap is a code feature that prevents drain water from siphoning back into the clean side, and the gurgle is drain water mixing with air on its way out. You will hear it most right after you draw a glass, when the system kicks on to refill the tank.
A constant hiss or a drain that runs nonstop is not normal. An RO system should send water to drain only while it is filling the tank, then shut off. A drain that never stops usually means one of three parts has failed: the automatic shutoff valve that is supposed to halt production when the tank is full, the check valve on the membrane outlet, or the flow restrictor on the drain line. The most common root cause is a tank that lost its air charge, because the system never reaches the pressure that triggers the shutoff valve, so it produces and drains forever. Check the tank charge before condemning a valve.
Spitting or dripping at the air gap onto the counter points the other way, to a partial clog in the drain line downstream of the air gap. Clear the line and the spitting stops. If the constant drain flow has you worried about waste, the RO water waste page explains the normal drain ratio and where the line is between normal and a fault.
Leaks at Fittings and Housings
Most RO leaks are at push-fit connections, and most of those are an incompletely seated tube.
For a drip at a push-fit fitting, depressurize the system, pull the tube out, cut the end clean if it looks scored, and push it firmly back in until it bottoms out. Then tug it hard. A connection that passes the tug test holds. If it still weeps, the collet or O-ring in that fitting may be worn, and the fitting is cheap to replace.
Leaks at a filter housing are usually the housing O-ring. After a filter change, a pinched or dry O-ring weeps around the threads. Back the housing off, seat the O-ring clean in its groove, a thin film of food-grade silicone grease helps, and hand-tighten with the housing wrench. Do not overtighten, since cracking a housing turns a drip into a flood.
Rising TDS and the Spent Membrane
This is the failure mode unique to RO, and a TDS meter is the only honest way to see it coming.
A healthy RO membrane rejects roughly 90 to 98 percent of dissolved solids. Systems certified to NSF/ANSI 58, the standard that covers point-of-use reverse osmosis, are tested for their TDS reduction and for the specific contaminants they reduce, and the rejection rate is a direct read on whether the membrane is still doing its job. To measure it, take a TDS reading on the feed water and another at the RO faucet, then divide the drop by the feed number. Feed of 300 ppm and faucet of 15 ppm is a 95 percent rejection, which is healthy.
When that number creeps down over months, the membrane is wearing out. Two things drive it. The membrane itself degrades, especially if chlorine in city water reached it because the carbon pre-filter was overdue, since free chlorine damages the standard polyamide membrane. Or the check valve on the membrane outlet has failed and lets pressurized feed water bypass the membrane during idle periods. Either way, more dissolved solids reach the faucet than should.
A separate and harmless effect is worth knowing so you do not panic over it. The first draw of the morning often reads higher than the rest. That is TDS creep, dissolved solids that diffuse across the membrane while the system sits idle and the tank water rests against it. The reading drops once you run the faucet and pull fresh permeate through. A small morning spike that clears is normal. A reading that stays high even after a full flush is a real decline. The what is TDS in water page explains what the number does and does not tell you about your water.
When rejection settles below roughly 85 to 90 percent and stays there after a flush, the membrane is spent and a new one restores performance. RO membranes generally last 2 to 5 years, while the sediment and carbon pre-filters need changing every 6 to 12 months to protect it. If your TDS reading bothers you because RO strips minerals along with contaminants, the does RO remove minerals page covers what that means for drinking water.
A Quick Symptom-to-Cause Table
| Symptom | Most likely cause | First fix |
|---|---|---|
| No water at all | Closed tank valve or low feed pressure | Open both valves, check pressure |
| Trickle or weak flow | Waterlogged storage tank | Drain tank, set air to 7 to 10 psi |
| Drain runs constantly | Tank lost charge or shutoff valve failed | Check tank pre-charge first |
| Gurgling at air gap | Normal during production | Nothing, unless it spits onto counter |
| Drip at a fitting | Unseated push-fit tube | Reseat tube, tug test |
| Rising TDS at faucet | Worn membrane or failed check valve | Measure rejection rate, replace membrane if low |
When to Replace Versus Repair
Pre-filters, push-fit fittings, housing O-rings, and tank air charge are all homeowner jobs that cost little. A membrane is a planned replacement part, not a repair, and you change it when rejection drops, not on a fixed date. The parts that justify a call are a failed check valve or automatic shutoff valve buried in the manifold on some all-in-one systems, where the valving is integrated and not user-serviceable.
For anything model-specific, the manufacturer’s manual is the right reference. Tank pre-charge specs, membrane part numbers, and shutoff valve locations vary between brands, so general advice only gets you so far. Comparing systems before you buy or replace is covered in best under-sink RO systems.
Test your water before treating it, and keep a TDS meter on hand so you can read rejection rate over time. Source water varies by region and well, and a before-and-after TDS reading is the fastest way to tell a working RO system from one that needs attention.