An under-sink reverse osmosis system normally sends some water to the drain while it produces filtered water. The problem starts when that drain flow continues after the storage tank should be full.

Do not judge the system immediately after filling several glasses, making ice, or changing filters. Those events can empty part of the tank and start another production cycle. Instead, give the system the full refill period stated in its manual, keep the RO faucet closed, and then listen near the drain connection. A continuing trickle, hiss, or steady flow deserves a closer look.

Confirm that the sound is coming from the RO system

Dishwashers, refrigerator ice makers, leaking faucets, and ordinary drain noise can be mistaken for RO discharge. Find the RO feed-water shutoff under the sink and close it without forcing the handle. If the drain sound stops, the RO system is the likely source. Reopen the valve after the check unless you plan to leave the system off.

Look for water around the filter housings, tubing connections, faucet base, storage tank valve, and drain saddle. A drain saddle that has shifted or clogged may create noise or leakage even when the amount of reject water is normal. Do not remove a pressurized housing or disconnect a drain tube just to see whether water is moving.

Check whether the system ever reaches a full tank

A standard tank system stops producing water when pressure on the filtered-water side becomes high enough to operate its automatic shutoff valve. Anything that prevents the tank from filling can keep reject water moving.

After the stated refill period, lift the storage tank slightly if it can be moved safely. A tank that still feels nearly empty points toward slow or failed production. Open the RO faucet and observe what happens. A useful initial stream that quickly fades can indicate a tank problem, while a weak stream from the beginning can point toward restricted production, low supply pressure, a closed valve, or blocked tubing.

Also check every fixture supplied by the RO system. A dripping RO faucet, leaking refrigerator connection, or ice maker that repeatedly calls for water can prevent the storage pressure from reaching the shutoff point. Dry each visible connection and use a clean paper towel to find small leaks that are hard to see.

Look for changes that happened before the continuous flow

The order of events is useful evidence. Write down whether the problem began after a filter change, plumbing repair, storage tank replacement, drain cleaning, or movement of the equipment under the sink.

After service, compare the tubing routes with the diagram on the system label or in its manual. A pinched tube, partially closed feed valve, filter installed in the wrong position, missing flow restrictor, or tubing connected to the wrong port can keep the system from shutting down correctly. If the installer changed any part, ask for the part number and an explanation of how it matches the original system design.

If nothing was recently changed, clogged prefilters are another possibility. Restricted feed flow can make the membrane produce water so slowly that the tank never reaches shutoff pressure. Use the replacement interval and pressure guidance from the system manufacturer rather than replacing filters solely because the drain is running.

Know which internal parts control shutdown

Several faults can create the same symptom. The automatic shutoff valve may not close. The membrane housing check valve may allow pressure to bleed backward. The drain flow restrictor may be damaged, missing, or incorrectly sized. The storage tank may have lost its air charge or developed a failed bladder. Low incoming pressure can also prevent a pressure-operated system from completing its cycle.

These parts should be tested as a system. Replacing the automatic shutoff valve alone may not solve the problem if a leaking check valve keeps pressure from building. Adding air to the tank without first emptying it and following the tank manufacturer's procedure can produce a misleading pressure reading.

Give the service company a specific symptom

Before scheduling service, record what you observed: whether drain flow continues after the expected refill period, whether closing the feed valve stops it, whether the storage tank becomes full, how the faucet flow behaves, and whether the problem began after another repair. A short video or audio recording can help document an intermittent sound.

Ask the technician to verify the incoming pressure, production rate, tank condition, automatic shutoff operation, check valve, flow restrictor, and tubing arrangement. Ask for the failed condition to be demonstrated before a part is replaced. After the repair, the system should be allowed to refill and its shutdown should be confirmed, not merely assumed because water comes from the faucet.

Know when to leave the system off

Close the RO feed valve and arrange service if water is leaking into the cabinet, a fitting will not stay dry, the drain connection is loose, or the system runs continuously despite having a full tank and no known water use. Keep the area dry and do not use electrical equipment in a wet cabinet.

If repair turns into a replacement decision, compare the equipment, service access, replacement-part availability, and documented shutdown test rather than accepting the first proposed system. The site's local water treatment ratings can help you decide which companies to question further.

The practical dividing line is simple: drain flow during tank refill is expected, but drain flow that never stops is a symptom. Confirm when it happens, determine whether the tank can fill, and have the complete pressure and shutdown path checked before replacing parts.