there's a musty smell coming from the washer, especially the rubber door seal, even after cleaning.

Diagnosis

The musty odor in the LG WM4000HWA is caused by the accumulation of a biological film (biofilm) consisting of undissolved detergent, fabric softener, and skin cells. This slime layer thrives in the dark, damp crevices of the rubber door gasket (bellows) and the outer tub. Because this model features a high-efficiency, low-water design, there isn't always enough water volume to flush out all the residue from the upper portions of the drum and the folds of the seal. The smell specifically clings to the rubber gasket because water pools in the bottom lip of the bellows after the spin cycle ends. If the door is closed immediately after a load, the interior becomes a humid incubator for mold and mildew. You can confirm this by pulling back the lower fold of the rubber seal and looking for a greyish-black slime or small black spots. If the smell persists after a Tub Clean cycle, it means the biofilm has thickened into a physical layer that chemicals alone cannot penetrate without mechanical scrubbing.

How to Fix It

To eliminate the odor and sanitize the system, follow this deep-cleaning sequence:

1. Manual Gasket Scrub: Mix a solution of 1 part white vinegar and 2 parts warm water. Soak a microfiber cloth and pull back the rubber bellows, wiping the entire circumference of the seal. Pay special attention to the bottom "sump" area where water collects and the holes where the water drains. Use an old toothbrush to scrub the tiny crevices where the rubber meets the metal drum.
2. Dispenser Drawer Purge: Press the release button on the detergent drawer and pull it completely out of the machine. Scrub the drawer itself and the internal housing (where the drawer slides in) using a damp cloth. Soap scum often builds up on the ceiling of the dispenser housing, dripping back into the drum and causing smells.
3. Drain Pump Filter Cleanout: Open the small access door at the bottom left of the front panel. Place a shallow tray or towel down, unscrew the small black drain hose to empty the residual water, then unscrew the large plastic debris filter (the pump filter) counter-clockwise. Remove any lint, coins, or hair, and rinse the filter under hot water.
4. Chemical Tub Clean: Close the door and the drawer. Add 2 cups of liquid chlorine bleach or a dedicated tablet cleaner (like Affresh) directly into the drum. Select the "Tub Clean" cycle from the dial. Ensure the machine is empty of clothes. This cycle uses a specific high-heat, high-water level agitation pattern designed to flush the outer tub.

If That Didn't Work

• Vinegar Shock Treatment: If bleach didn't work, the buildup may be mineral-based. Run a Tub Clean cycle with 2 cups of distilled white vinegar. Do not mix vinegar and bleach in the same cycle, as this creates toxic chlorine gas. Run one rinse cycle with plain water after the vinegar wash to neutralize the scent.
• Door Seal Replacement: If you see permanent black spotting (mold) that cannot be scrubbed away, the mold has permeated the porous rubber of the bellows. You will need to replace the door gasket (Part No: ANC11321101 or similar depending on specific sub-model). This requires removing the top plate and the front panel to access the spring clamp.
• External Venting Check: Ensure the drain hose is not pushed too far into the standpipe. If the hose is submerged in water in the wall pipe, "siphoning" can occur, pulling dirty wastewater back into the drum after the cycle ends, which creates a permanent sewage-like smell.
• High-Heat Sanitization: Run a load of old towels on the "Heavy Duty" setting with the temperature set to "Hot." The sustained high heat can sometimes break down biofilm that a shorter Tub Clean cycle misses.

Heads Up

• Avoid Fabric Softeners: Liquid fabric softeners are a primary cause of biofilm. They are essentially wax and oil that coat the outer tub. Switch to distilled white vinegar in the softener compartment for a natural softener that actually cleans the machine.
• The "Open Door" Rule: Never close the door completely when the machine is not in use. Leave it cracked at least 2 inches to allow airflow to dry the gasket.
• Drawer Ventilation: Pull the detergent drawer out by one inch after every wash. This prevents mold from growing in the ceiling of the dispenser housing.
• Detergent Volume: Use only 2 tablespoons of HE (High Efficiency) detergent per load. Over-sudsing leads to residue buildup in the outer tub, which feeds mold growth.