Troubleshooting The airflow from my Philips purifier has weakened over time If the air coming out of your Philips Air Purifier has weakened over time, chances are that the pre-filter is dirty. To clean it, please follow the steps below. Cleaning the pre-filter Switch off and unplug the air purifier. Pull the bottom part of the front panel and gently lift to remove it. Press the clip down (1) and pull the pre-filter towards you. Use a soft brush to brush away any dust and wash the pre-filter under running water. Allow the pre-filter to air dry thoroughly before placing it back in the Air Purifier. Re-attach the front panel by pressing the top part of the panel on to the top of the Air Purifier and gently push it against its body. Connect the Air Purifier to a power source. Touch the Child Lock button for 3 seconds to reset the pre-filter cleaning time. Note These instructions do not apply to model AC1211.