Water purifier diagnostics
Your RO water tastes bad — here is how to find out why
In most cases it is an exhausted activated-carbon filter — a ₹200-450 part that is the single most common cause of taste and odour complaints, and one that many households never replace because the machine keeps running without it.
Important
Before you open anything
- Switch off and unplug the purifier at the mains before opening any housing. These units sit near a water supply and the SMPS carries mains voltage.
- Never look directly at an operating UV lamp. UV-C causes painful corneal burns that appear hours later.
Does this match your symptoms?
- Water tastes flat, chemical, plasticky or of chlorine
- A musty or stale smell, especially first thing in the morning
- A salty or metallic taste that was not there before
- Taste changed gradually over weeks rather than suddenly
Work through the causes in this order
Ordered by how often each turns out to be the real cause — not by what it costs to fix. 1 of these costs nothing to resolve.
- Most commonYou can fix this₹200 – ₹450
1. Exhausted activated-carbon filter
How to confirm it
Check when the carbon filter was last changed. If it is more than 6-8 months, or you cannot remember, this is almost certainly it. Carbon does not block water when it is spent — it simply stops adsorbing, so the machine gives no warning at all. A chlorine or chemical taste that crept in over weeks is the classic signature.
The fix
Replace the activated-carbon (pre-carbon and post-carbon) cartridges. On most domestic units this is a ten-minute job with a filter spanner. Run 10-15 litres to drain afterwards to flush carbon fines, which is why the first glass often looks faintly grey.
Part: Pre-carbon and post-carbon cartridge
- CommonYou can fix thisFree to fix
2. Stagnant water sitting in the tank
How to confirm it
Did the taste appear after a holiday, a long weekend or any period of low use? Draw a glass, then drain the tank completely and draw another. If the second glass tastes clean, it was stagnation.
The fix
Drain the tank fully and let it refill. Where the unit has an in-tank UV steriliser, check it is actually working. Make a habit of draining the tank before leaving home for more than three or four days.
Part: None — this one is free
- CommonTechnician job₹500 – ₹1,200
3. Failed UV lamp allowing biofilm
How to confirm it
UV lamps stop emitting useful ultraviolet after roughly 6,000-9,000 hours — about a year of continuous use — while still glowing visibly. A glowing lamp is NOT a working lamp. If the smell is musty or earthy and returns within days of draining the tank, suspect biofilm downstream of a dead lamp.
The fix
Replace the UV lamp and have the tank and post-UV plumbing sanitised. Replacing the lamp alone without sanitising leaves the existing biofilm in place, and the smell comes back.
Part: UV lamp, plus tank sanitisation
- OccasionalTechnician job₹1,500 – ₹3,500
4. Failing or bypassed RO membrane
How to confirm it
This is the one worth ruling in or out properly, because it is the expensive cause. Measure the TDS of your purified water with a meter. If it has climbed well above its usual figure — say from 60 ppm to 250 ppm — the membrane is passing dissolved solids it used to reject. A salty or brackish taste with a high output TDS is close to conclusive.
The fix
Replace the RO membrane. Before you do, ask the technician to check the sediment and carbon pre-filters as well: a choked pre-filter is the most common reason a membrane dies early, and fitting a new membrane behind a clogged pre-filter simply repeats the failure.
Part: RO membrane
- OccasionalTechnician jobUp to ₹400
5. TDS controller set too high
How to confirm it
Applies only to units with a TDS controller or mineral blender. If a technician recently serviced the unit, the blend valve may have been left open too far, mixing in more untreated water than before. Output TDS will be high but the membrane itself is healthy — the giveaway is that the change was sudden and coincided with a service visit.
The fix
Have the TDS controller readjusted to give an output of roughly 50-150 ppm. This is a valve adjustment, not a part replacement, and you should not be charged for a component.
- RareTechnician job₹3,000 – ₹15,000
6. Hydrogen sulphide in the incoming supply
How to confirm it
A distinct rotten-egg or sulphurous smell, usually stronger in borewell supply and often worse in the morning. Fill a glass straight from the inlet tap, before the purifier. If it smells there too, the purifier is not the problem.
The fix
This needs treatment upstream of the purifier — typically oxidation and filtration, or an activated-carbon pre-treatment stage sized for the job. A domestic RO alone will not resolve it, and replacing its filters repeatedly will only waste money.
Part: Pre-treatment stage
When to stop and call a technician
- The taste is salty and your output TDS has risen sharply — get the membrane and pre-filters assessed together, not the membrane alone
- The smell returns within a week of a tank drain and filter change
- You smell rotten eggs, which points at your supply rather than the machine
- The unit is under warranty — opening it yourself may void it, and the filter change should be free
Costs shown are indicative bands for India, published so you can sanity-check a quote — they are not quotations, and they vary by brand, city and whether the technician is brand-authorised. Last reviewed 2026-08-08.