Frequently Asked Questions What do the codes on my Philips Air Purifier mean? Your Philips Air Purifier gives you reassurance by displaying real-time indoor air quality codes. Discover what the codes mean. Meanings of codes in Philips Air Purifiers Depending on the Air Purifier model you have, you might find one or more of the following indicators or codes: Code: A Auto mode Code: C Gas mode (AC3254 and AC3256 only) Code: L Child lock on Code: UL Child lock off Code: F0 Clean the pre-filter Code: A0 to A9 Replace NanoProtect S3 HEPA filter Code C0 to C9: Replace NanoProtect S3 AC filter Code SL Sleep Mode Code 1h: Timer mode Code t: Turbo Mode. Error Code: Er The hall sensor is not working properly. Remove and then reattach the front panel. If the error code “Er” still persists, we recommend you to contact us. Error code: E1 to E4 The appliance has malfunctions. In this case, we recommend you to contact us.