On the Coway Airmega 400 there are two separate reminders, each with its own reset. After washing the pre-filters, touch and hold Pre Wash for 2–3 seconds; after replacing the Max2 filters, touch and hold Max2 Change for 2–3 seconds. The same two buttons do the job on the Wi-Fi Airmega 400S. A reminder that keeps returning usually means the wrong button was held, or one side of the purifier was skipped.
Do this first
These steps follow Coway's user manual for the Airmega 400S, 400, 300S and 300. The 400 cleans air through both sides, so every job below happens twice, once on the left and once on the right.
- Switch off and unplug. Coway warns against changing filters while the purifier runs.
- Pull off both side covers by gripping the top of each and pulling it away from the body.
- Remove both pre-filters. Press the two tabs at the top of each mesh panel and pull it outward.
- For a Pre Wash light: vacuum the pre-filters, or rinse them gently if vacuuming leaves dust behind. Dry them in the shade before refitting.
- For a Max2 Change light: lift out both Max2 filters, unwrap the new pair without scratching the media, and set each in its case with the green side facing the purifier.
- Refit in reverse order: Max2 filters, then pre-filters, then covers. Seat each cover in the grooves at the bottom first, then push it into place.
- Plug in and turn it on. Touch Power and you'll hear a chime.
- Touch and hold the lit button for 2–3 seconds: Pre Wash after cleaning, Max2 Change after a filter swap. If you did both jobs, reset both buttons.
What the filter readout is telling you
Beside each button, the 400 and 400S show remaining life in steps of 100, 80, 60, 40, 20 and 0 percent. The lettering on the button itself lights up only when that job is due. Coway says the purifier gauges how much air has passed through it rather than simply counting days, so the reminder arrives sooner when Smart mode spends long stretches on the higher fan speeds.
Coway's baseline is about 2 weeks between pre-filter cleanings and about 12 months between Max2 changes, both worked out at 8 hours a day on maximum speed. On the 400S the pre-filter interval can be changed in the Airmega app; the 400 has no app, so its schedule is fixed.
Why the light stays on or the purifier won't start
- The wrong button got the hold. A Max2 Change light ignores a press of Pre Wash, and the reverse.
- The touch was too brief. These are touch keys, and Coway asks for a 2–3 second hold. A tap won't clear anything.
- A side cover isn't seated. Coway's manual says the 400 won't turn on if the left and right covers are fitted incorrectly. Hook the bottom edge in first, then press the top home on both sides.
- The new Max2 filters are still wrapped. Every new pair comes in plastic that has to come off before use. Coway puts this in its quick-start steps, and it's an easy miss in a hurry.
- A filter is facing backwards. Green side toward the purifier. A reversed filter may still fit, but Coway says it can shorten its life.
If the colored ring on the front is what worries you rather than the filter buttons, that's the air quality indicator. On the 400 it runs blue, purple, pink and red, and on the 400S it shows blended pairs such as blue and green. Coway's fix for a reading that never moves is to clean the pollution sensor behind the front cover every 2 months with a dry cotton swab, never water.
When to call Coway
If neither button responds to a 2–3 second hold, or the purifier won't power on with both covers properly fitted, use the contact options on Coway's Airmega 400/400S support page. The same applies to a strange sound, smell or smoke, which Coway says calls for unplugging straight away. Coway's manual says repair work belongs with a qualified technician, and that repairs it doesn't recommend can void the warranty.
Filters and what they cost
Coway's Max2 Filter Set for the Airmega 400 is a pair, one per side, and it was $129 at Coway when we checked on 2026-10-01. With a set lasting up to about a year, that's roughly $129 a year, or about $387 over three years, before electricity. The filter cost calculator adds power use. That bill is the 400's biggest ongoing cost, and our Airmega 400 vs Levoit Core 600S comparison shows how it stacks up against a cheaper big-room rival.
Owners with a smaller Coway as well will find its single-cartridge routine in the Mighty2 filter reset guide. For the wider lineup, including whether the 400S's app is worth the extra $100, see our Coway Airmega review.
Frequently asked questions
Why does the Airmega 400 Pre Wash light come on so often?
Coway sets the pre-filter cycle at roughly 2 weeks, based on 8 hours a day at maximum speed. Washing or vacuuming both mesh pre-filters and holding Pre Wash for 2–3 seconds restarts it. On the Wi-Fi 400S the cleaning interval can be changed in the app.
Do I need to replace both Max2 filters on the Airmega 400?
Yes. The 400 filters from both sides and Coway's instructions replace the Max2 filters on both sides at once, which is why the Max2 Filter Set comes as a pair. A single fresh filter would leave half the airflow running through spent media.
Which way round does the Max2 filter go in?
Coway says to place each Max2 filter in its case with the green side facing the purifier. Reversing it can affect how long the filter lasts. Check the side marked FRONT on each filter before refitting the pre-filter and cover.
Can I wash the Max2 filters?
No. Coway says the Max2 filters are not reusable and must not be cleaned with water. Only the two mesh pre-filters are washable, and they should dry in the shade before going back in.
Is the Airmega 400S reset different from the Airmega 400?
The buttons are the same: Pre Wash and Max2 Change, each held for 2–3 seconds. The 400S adds a Wi-Fi button and app, which also shows filter status and lets you change the pre-filter cleaning interval. The 400 has a Timer button in place of Wi-Fi.
