DIAGNOSIS The solid-state formaldehyde sensor is likely being triggered by other volatile organic compounds (VOCs), not necessarily formaldehyde. It is a broad VOC indicator.
FIX Manually activate the 'Deep Clean' cycle. On the machine or in the app, select the Deep Clean mode. It runs the fan at maximum for up to 30 minutes. This clears the sensor chamber of accumulated compounds that can cause false high readings. After the cycle, check the reading again in a ventilated room.
IF THAT DIDN'T WORK Common household items emit VOCs. Remove potential sources from the room: aerosol sprays, strong cleaning chemicals, nail polish remover, new furniture, or certain candles. Run the purifier with these removed for 24 hours.
HEADS UP The formaldehyde number on the display is an estimate, not a laboratory-grade measurement. It is sensitive to alcohols and esters found in many products. A consistently high reading in an empty room may require a sensor reset via Dyson Support.