If you notice your Dyson purifier displaying a 999 Severe reading, and there’s no obvious source of pollutants within your indoor environment that could be causing this, the reading is likely to be due to a blockage in the machine’s sensor port(s).
Clearing the sensor ports of any dust, fluff or debris is quick and easy - our ‘Cleaning the sensor’ article shows you what to do!
Further instructions on cleaning your Dyson purifier can be found in your User guide, as well as within the MyDyson™ app under 'Using your machine' and then choosing 'Cleaning your machine'.
If this reading is still showing after cleaning the sensor and the machine, we’d suggest following the information in the 'Power Cycling/Restarting' your machine article.