my BMW 3 Series E90/E91 has a musty smell when running the AC, what could it be?

Solution

1. Preparation

• Gather tools and parts before beginning the repair.
• Ensure the vehicle is parked on a level surface and the engine is turned off.
• Disconnect the negative battery terminal to prevent electrical issues.

2. Replace Cabin Air Filter

• Locate the cabin air filter housing, typically under the dashboard on the passenger side.
• Remove the cover panel by unscrewing any retaining screws or unclipping it.
• Take out the old cabin air filter and inspect for mold or dirt.
• Install a new cabin air filter, ensuring it is oriented correctly.
• Reattach the cover panel.

3. Clear AC Drain Tube

• Locate the AC drain tube, usually found on the firewall under the hood.
• Inspect the drain tube for any blockages, such as dirt or debris.
• Use compressed air to blow out any obstructions or carefully insert a flexible wire to clear the drain.
• Ensure that the tube is securely reattached and free from leaks.

4. Clean Evaporator Core

• If accessible, remove the dashboard cover to reach the evaporator core.
• Spray an AC cleaner or coil cleaner onto the evaporator core to eliminate mold and bacteria.
• Allow the cleaner to penetrate for the recommended time before rinsing with water.
• Reassemble any components removed during this process.

5. Reconnect Battery and Test

• Reconnect the negative battery terminal.
• Start the vehicle and run the AC on high for several minutes to check for odors.
• Monitor the system for proper airflow and ensure the musty smell has dissipated.