Common Faults in Domestic RO

If your domestic 5-stage RO pump does not switch off, it’s a very common fault and almost always one of a few components. I’ll go step-by-step, starting with the most likely causes and what you can safely check at home.

🔴 MOST COMMON CAUSES (90% of cases)

1️⃣ Faulty Auto Shut-Off Valve (ASO)

This is the #1 cause.

What it does:

The ASO valve stops water to the membrane when the tank is full.

Symptoms of failure:

Pump keeps running

Drain water keeps flowing

Tank is already full or pressurised

How to test:

Close the tank ball valve

Leave the RO running for 2–3 minutes

👉 If the pump keeps running → ASO valve is faulty.

✅ Solution: Replace ASO valve (cheap and easy)

2️⃣ Low or Failed Storage Tank Pressure

If the tank bladder has low air pressure, the system never senses “full”.

Correct tank pressure (empty):

7–10 psi (0.5–0.7 bar)

Check:

Turn off water supply

Empty the tank completely

Use a tyre pressure gauge on tank valve

Problems:

0–2 psi → pump never shuts off

Bladder burst → tank feels heavy even when “empty”

✅ Solution:

Pump air to 7–10 psi

Replace tank if bladder is damaged

3️⃣ Blocked or Incorrect Flow Restrictor

If waste water flow is restricted incorrectly, pressure never builds.

Symptoms:

High drain flow or almost no drain flow

Low product water

Check:

Flow restrictor must match membrane (e.g. 300 ml/min for 75 GPD)

✅ Solution: Replace correct flow restrictor

🟡 SECONDARY CAUSES

4️⃣ Leaking Faucet or Check Valve

A tiny leak keeps pressure from building.

Check:

RO faucet dripping?

Check valve stuck open?

✅ Solution: Replace check valve or faucet

5️⃣ Faulty Pressure Switch (if pump system)

Most pump systems use:

Low-pressure switch

High-pressure switch

If the high-pressure switch fails, the pump never stops.

Test:

Disconnect tank line briefly

If pump never cuts → switch faulty

✅ Solution: Replace high-pressure switch

🛠 QUICK DIAGNOSIS ORDER (Do this)

Close tank valve → does pump stop?

Check tank air pressure

Check ASO valve

Check flow restrictor

Check pressure switch

⚠️ Important

❌ Do not let the pump run continuously

It will:

Overheat the pump

Destroy the membrane

Waste large amounts of water

If you want, tell me:

Does your system have a booster pump?

Tank size (8L / 12L / 18L)?

Membrane rating (50 / 75 / 100 GPD)?