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Air Conditioner Smells Musty or Sour? Clean the Drain Pan

A smelly AC usually means mildew or slime in the damp parts. Replace the filter, clean the pan and drain, then keep it from growing back.

By Jamilur Rahman Updated November 5, 2026 · 6 min read

Time
45 min
Cost
Under $20
Difficulty
Medium
An air conditioner with its front panel open and a dirty gray filter pulled out beside it
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. Match the smell to the cause
  9. What to skip
  10. FAQ
  11. Related problems

Quick answer

Why does my air conditioner smell? An AC makes water as it cools, and when that water sits in the filter, the coil or the drain pan, mildew and slime grow and give off a musty, sour or vinegar smell. Replace the filter, clean out the drain pan and let the unit dry out. Drain pan tablets slow the regrowth, so the smell stays away.

Free fix vs. permanent fix

Free fix · 30 minutes

Clean the filter and the drain pan

  • Turn the power off, then wash or replace the filter
  • Empty and scrub the drain pan with vinegar and water
  • Pour vinegar through the drain line if it has an access port
  • Run the fan alone for 15 minutes after cooling to dry the coil

How long it lasts: A few weeks to a season. The damp pan grows slime again if nothing stops it.

Most readers choose Permanent fix
AC drain pan tablets (illustration)
Illustration

Drain pan tablets

The pan is damp every time the AC runs, so slime grows back after you clean it. A tablet sits in the pan and keeps working between cleanings. It does not replace the filter or coil cleaning.

What to get: AC drain pan tablets

How long it lasts: A few months per tablet, as the package says. They slowly release a cleaner that keeps slime from forming in the pan.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Screwdriver or nut driver

    For the access panel on some units

    You have it

  • White vinegar and a soft brush

    For scrubbing the pan

    You have it

  • Wet-dry shop vacuum

    To pull out standing water

    You have it
  • Replacement air filter (illustration)
    Illustration

    Replacement air filter

    Match the size on your old filter

    Check price
  • Our pick
    AC drain pan tablets (illustration)
    Illustration

    AC drain pan tablets

    Drop into the pan to stop slime from growing

    Check price
  • No-rinse foam coil cleaner (illustration)
    Illustration

    No-rinse foam coil cleaner

    For a window unit's coil that smells

    Check price

Why it happens

1

A dirty, damp filter

Dust and moisture on the filter feed mildew. The first air through it carries the smell into the room.

2

Standing water in the drain pan

Water collects in the pan under the coil. Stagnant water grows bacteria and mold, which give a sour or vinegar smell.

3

A clogged drain line

A slime clog keeps water in the pan. On central systems it can back up and add to the smell.

4

A moldy evaporator coil

The coil is cold and wet. Dust sticks to it and mildew grows, so the smell is strongest when you first turn the AC on.

5

Something that does not belong

A dead mouse in a duct or a window unit can smell like urine or rot. A sharp, hot smell may be an electrical problem.

Try this first (free)

Do these in order, starting with the simplest.

  1. Turn off the power

    Switch the unit off and unplug it, or turn off the breaker for a central system, before you open anything.

  2. Replace or wash the filter

    Tool used: Replacement air filter

    Slide out the filter. If it is a washable one, rinse it with warm water and mild soap, and let it dry fully. A disposable one gets replaced.

    Replacement air filter (illustration)
    Illustration

    Tool for this step

    Replacement air filter · Match the size on your old filter

    Check price on Amazon

  3. Clear and scrub the drain pan

    Tool used: White vinegar and a soft brush

    On a central system, the pan is under the indoor coil, often behind an access panel. On a window unit, it is the base under the coil, and the manual shows how to reach it. Pull out standing water with a shop vac, then scrub with a vinegar and water mix.

  4. Flush the drain line

    If your central system has a drain access port, pour in about a quarter cup of vinegar. It helps clear slime, and your installer can show you the port.

  5. Drop in a drain pan tablet

    Tool used: AC drain pan tablets

    Place a tablet in the clean pan as the package says. It will release a cleaner slowly as water passes over it.

    AC drain pan tablets (illustration)
    Illustration

    Tool for this step

    AC drain pan tablets · Drop into the pan to stop slime from growing

    Check price on Amazon

  6. Dry the coil after each use

    On a window unit with a fan-only setting, run it on fan for 10 to 15 minutes before you turn it off. It dries the coil and cuts mildew.

When free fixes aren't enough

If the smell is back within a couple of weeks, the pan is growing slime faster than cleaning removes it, and a tablet is what keeps it down. If the smell comes from the coil itself, a window unit needs a foam coil cleaner, and a central system needs a technician, because the coil is not safe to open yourself. Stop and call a pro if you smell burning, hot plastic or a sharp electrical smell. If you smell rotten eggs, leave the house and call your gas company.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
AC drain pan tablets (illustration)
Illustration
AC drain pan tablets
slime and mildew growing in the drain pan, which causes the musty or sour smell Anyone who cleaned the pan and wants the smell to stay away. Check price
Upgrade pick
No-rinse foam coil cleaner (illustration)
Illustration
No-rinse foam coil cleaner
a moldy coil that smells when the AC starts Window unit owners whose smell is strongest on startup. Check price
Best pick
AC drain pan tablets (illustration)
Illustration

AC drain pan tablets

Fixes: slime and mildew growing in the drain pan, which causes the musty or sour smell

Look for: Made for AC condensate drain pans, not for toilets; Slow-dissolving, with a stated number of months; Safe for the metal and plastic parts of your system.

Who it's for
Anyone who cleaned the pan and wants the smell to stay away.
Honest downside
Your unit has no accessible pan or the manual says not to add chemicals. Clean it by hand.
Check price on Amazon
Upgrade pick
No-rinse foam coil cleaner (illustration)
Illustration

No-rinse foam coil cleaner

Fixes: a moldy coil that smells when the AC starts

Look for: A no-rinse formula that drains away as the unit runs; Safe on aluminum fins, and stated on the label; A spray that clings to the coil.

Who it's for
Window unit owners whose smell is strongest on startup.
Honest downside
You have a central system. Leave the coil to a technician.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on HVAC maintenance guides, manufacturer instructions for window and central air conditioners, and the problems and fixes people report in reviews. We do not test products by hand.

Match the smell to the cause

  • Musty or mildew: a damp filter, coil or pan. Clean them.
  • Sour or vinegar: bacteria in standing pan water, or a clogged drain.
  • Urine or ammonia: stagnant water, or a dead mouse in the unit or ducts.
  • Burning or hot plastic: an electrical problem. Turn it off and call a pro.
  • Rotten eggs: possible gas. Leave the house and call your gas company.

What to skip

Spray fresheners and scented filters

Scented sprays and filters cover the smell for a day and let mildew keep growing. The smell comes back, and perfume mixed with mildew smells worse.

Frequently asked questions

Why does my air conditioner smell musty?
Mildew in the filter, the coil or the drain pan. The air passing over a damp, dusty surface carries the smell. A dirty filter and a standing pan of water are the first things to check.
Why does my air conditioner smell like pee or vinegar?
Bacteria in stagnant pan water can smell sour or like ammonia, and mold can smell like vinegar. A dead rodent can smell like urine. A sharp vinegar or hot smell that is not damp can be an electrical part, so if it is strong or you see smoke, turn it off and call a pro.
Why does my air conditioner smell weird only when it starts?
Mildew builds up while the unit is off and the coil stays damp. Cleaning the coil and filter, and drying the unit with the fan after cooling, helps.
How do I stop an air conditioner from smelling for good?
Keep the filter clean, keep the drain pan and line clear, let the coil dry after use and add a drain pan tablet. A yearly service on a central system helps too.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

AC drain pan tablets (illustration)
Illustration

Our pick: AC drain pan tablets

Check price