Delhi's smog season is when a car's cabin air system earns its keep. Air quality in the capital slipped into the "poor" category on 21 October 2026, and the Commission for Air Quality Management invoked Stage I of the Graded Response Action Plan (GRAP) across NCR the same day, All India Radio reported. If you're buying a used car now, the cabin air filter, AC, heater and demisters deserve as much attention as the engine.
The short version: check when the cabin air filter was last changed (or pull it out and look), make sure the AC, heater and recirculation work properly, and test the windscreen and rear-window demisters. A clogged filter is cheap to replace, but a failed AC or heater can be costly.
Smog-season checks at a glance
- Cabin air filter: ask for the last replacement date; a grey, clogged filter needs changing.
- AC and fan: strong, steady airflow on every fan speed, with no musty smell.
- Recirculation: the fresh-air/recirculation switch should change the airflow source.
- Heater: warm air once the engine has warmed up.
- Demisters: the windscreen should clear quickly in defrost mode; the rear demister should clear the back glass.
- Air purifier (if fitted): check that it powers on.
Why the cabin air system matters in Delhi
A closed, air-conditioned car can cut your exposure to traffic pollution significantly. A peer-reviewed study of Delhi commutes by Goel and colleagues, published in Atmospheric Environment in 2015, found PM2.5 concentrations were about 50% lower inside an air-conditioned car than outside, while cars with open windows saw levels about 30% higher than outside (study PDF). That benefit depends on a working AC system and a filter that isn't clogged.
1. Check the cabin air filter
The cabin air filter (Honda calls it the dust and pollen filter; Kia calls it the climate control air filter) cleans the air coming into the cabin. In many cars it sits behind the glovebox and is easy to inspect.
How often it should be changed depends on the car:
- Kia: the Indian owner's manual schedule lists an inspection at 12 months or 10,000 km and replacement at 24 months or 20,000 km, whichever comes first. Kia's manual adds that cars driven for long periods in "severely air-polluted cities or on dusty rough roads" should have the filter checked more often and replaced sooner (Kia owner's manual).
- Honda: the WR-V maintenance schedule says to replace the dust and pollen filter every 20,000 km or one year.
What to do on a used car:
- Ask the seller for the last service invoice and look for a cabin filter replacement.
- Open the glovebox and inspect the filter if you can. A clean filter is white or light grey; a dark grey or black filter packed with dust and leaves is overdue.
- Price a replacement into your offer if it hasn't been changed recently, especially for a car that has lived through a Delhi winter.
Honda's manual says to replace the filter sooner if airflow from the heating and cooling system "deteriorates noticeably" or the windows "fog up easily". Both are useful clues on a test drive.
2. Test the AC, fan and airflow
- Start the car and run the AC on full with the fan on each speed. The airflow should be strong and steady from all vents.
- Check that cold air comes through within a few minutes of driving.
- Sniff the air from the vents. A musty smell can point to a dirty filter or moisture in the system.
- Listen for rattles, whines or clicks from behind the dashboard when you change fan speeds or modes.
Weak airflow at every setting can mean a clogged filter or a blower problem; good airflow but no cooling usually points to the AC system itself and needs a workshop check.
3. Check recirculation and fresh-air modes
On smoggy days, recirculation helps keep outside fumes out, but it isn't meant to stay on all the time. Kia's Indian manual warns that continued recirculation "may allow humidity to increase inside the vehicle which may fog the glass and obscure visibility", and can cause drowsiness, and advises using fresh air "as much as possible whilst driving" (Kia owner's manual).
On the test drive, switch between the two modes and check that the change is audible and that the airflow behaves as expected. A stuck flap is usually fixable, but worth knowing about before you pay.
4. Test the heater
Car heaters use heat from the engine's coolant, as Honda's manual explains, so the heater only works properly once the engine has warmed up. After 10 to 15 minutes of driving, turn the temperature to hot: the air should be clearly warm. A heater that stays lukewarm can point to a cooling system problem, such as low coolant or a faulty thermostat, which should be checked before you buy. Our guide to warning signs of engine problems covers other cooling-system clues.
5. Test the demisters
Foggy winter mornings make demisters a safety item, not a luxury.
- Windscreen: select the defrost mode. Kia's manual says that in this mode the system automatically switches to fresh air with the AC on, and for maximum defrost the temperature should be set to hot and the fan to the highest speed. The screen should start clearing within a few minutes.
- Rear window: switch on the rear demister and check that the heating lines clear the glass evenly. Kia notes the rear demister works only while the engine is running and switches off after about 20 minutes. Broken heating lines leave strips of fog.
If the windscreen fogs up quickly and clears slowly even in defrost mode, suspect a clogged cabin filter or an AC problem.
6. Check any built-in air purifier
Some newer cars come with factory air purifiers or upgraded filters. For example, Kia lists a "Smart Pure Air Purifier" on the Sonet, and MG says all its models come with PM2.5 air filters. If the car you're looking at has a purifier, make sure it switches on and runs, and ask whether its filter has been replaced as the manual recommends. Treat the purifier as a bonus rather than a reason to pay much more.
7. Don't forget the smog-season paperwork
Pollution season also brings tighter checks on paper:
- Delhi enforces "No PUC, No Fuel", so the car needs a valid PUC certificate. See our guide to PUC rules for used cars.
- GRAP stages can restrict older cars, especially BS-III petrol and BS-IV diesel, when air quality worsens. Read our guide to GRAP rules for BS-IV cars.
- Cars registered outside Delhi face entry curbs if they aren't BS-VI. Our guide to Delhi's non-BS6 entry ban explains what that means.
A five-minute cabin air checklist
- Ask when the cabin filter was last replaced and check the invoice.
- Inspect the filter behind the glovebox if it's accessible.
- Run the AC and fan through every speed and sniff for odours.
- Switch between fresh air and recirculation.
- Warm up the engine and test the heater.
- Test the windscreen defrost and the rear demister.
- Check that any air purifier works.
Common mistakes to avoid
- Testing the AC only on a short, cold-engine drive.
- Ignoring a musty smell as "just an old car smell".
- Leaving recirculation on all the time, which can fog the windows.
- Assuming a purifier makes up for a clogged cabin filter.
- Forgetting to check the rear demister before winter.
FAQs
How often should a car's cabin air filter be changed?
It varies by maker. Kia's Indian schedule says replace every 24 months or 20,000 km and inspect at 12 months or 10,000 km, while Honda says every 20,000 km or one year. Change it sooner if you drive in heavy pollution or dust.
Does the car's AC protect against Delhi pollution?
It helps. A 2015 study of Delhi commutes found PM2.5 levels about 50% lower inside an air-conditioned car than outside, but a clean filter and closed windows matter.
Should I use recirculation mode in smog?
It helps keep fumes out in heavy traffic, but carmakers advise using fresh air as much as possible while driving, because prolonged recirculation can raise humidity, fog the windows and cause drowsiness.
Why do my car windows fog up so quickly?
A clogged cabin filter, too much recirculation or an AC problem are common causes. Use the defrost mode, which turns the AC on to dry the air.
Is a car air purifier worth paying extra for in a used car?
It's a useful bonus, but check that it works and don't pay a big premium for it.
The bottom line
In smog season, a used car's cabin air system is part of your health, not just your comfort. Check the cabin filter, AC, heater, recirculation and demisters on every car you shortlist, and budget for a new filter if it's overdue. Pair this with our checks for the battery, tyres and airbags, our winter car care tips for Delhi NCR, and then browse used cars in Delhi NCR on Hooter.