Why Is My Dyson Purifier Hot+Cool Formaldehyde Not Detecting VOCs?
- Purifier needs ~20 min warmup for the HCHO sensor
- App display mode set to show only PM2.5, not VOC
- HCHO sensor in calibration after a new carbon filter
Problem Description
Your Dyson Purifier Hot+Cool Formaldehyde HP09 is not showing VOC or formaldehyde readings in the MyDyson app even when you are in a freshly painted room or near cleaning products with high VOC content. The air quality display shows blank or zero for VOC and HCHO readings. Sensor warmup time, incorrect display mode selection in the app, and sensor calibration state after a new filter installation are the most common causes of missing VOC readings.
Why This Happens in Real Homes
The formaldehyde (HCHO) sensor on the Dyson HP09/TP09 is a solid-state sensor that works differently from the particle and VOC sensors, and most 'not detecting' reports are actually normal warmup and calibration behavior rather than a fault. Two timing facts explain the majority of cases: the HCHO sensor needs about 20 minutes of runtime each session to reach operating temperature before it produces a reading, and after first use (or a new activated-carbon filter) it needs 3-5 days of continuous operation to establish its baseline. So a blank or N/A HCHO value on a freshly set-up machine, or right after startup, is expected - give it time before concluding anything is broken.
The other common causes are display and environment settings. The MyDyson app can be set to show only certain metrics, so VOC and HCHO may simply not be selected in the air-quality view - switching the display mode reveals them, and updating the app ensures all sensor readings appear after firmware changes. On the environment side, heavy ventilation dilutes VOCs before they reach the sensor, so testing next to an open window will show low readings that are technically accurate. Make sure both the carbon and HEPA filters are correctly installed (the carbon layer is part of the sensing/filtering path), and confirm your specific model actually has the formaldehyde sensor, since only the HP09/TP09-class machines include it.
Symptoms
- App shows zero/blank for VOC and formaldehyde
- Air-quality screen doesn't display an HCHO value
- VOC reading never changes near strong odors
- Formaldehyde reading shows N/A
- Readings appeared briefly then disappeared
- Other readings show but VOC/HCHO are missing
- No HCHO value after a new filter
- VOC display seems absent
Recognize these? Here's what usually causes it.
Common Causes
- Purifier needs ~20 min warmup for the HCHO sensor
- App display mode set to show only PM2.5, not VOC
- HCHO sensor in calibration after a new carbon filter
- Room ventilation too high, diluting VOCs before they reach the sensor
- Firmware update changed the display; app needs updating
- HCHO sensor needs 3-5 days of continuous operation for a baseline
- Filters not correctly installed
- Sensor still in its post-install calibration state
Most fixes happen in the first 3 steps.
Do not attempt to clean or remove the HCHO sensor module inside the Dyson HP09. The electrochemical sensor is factory calibrated and user access to it voids the warranty and permanently damages the calibration.
Tools & Requirements
These tools will help you complete this fix.
As an Amazon Associate we earn from qualifying purchases.
Step-by-Step Solution
Allow 20-Minute Warmup for HCHO Sensor
The Dyson formaldehyde electrochemical sensor requires approximately 20 minutes of operation to reach its operating temperature before generating reliable readings. If you have just powered on the purifier or just replaced the filter give it at least 20 minutes of continuous operation before checking VOC and HCHO readings in the MyDyson app. The sensor icon in the app may show dashes or loading symbols during the warmup period which is normal.
Switch App Display to VOC and HCHO View
Open the MyDyson app and tap the HP09 purifier. On the air quality screen swipe left or right through the sensor reading tiles. The display cycles through PM2.5, PM10, VOC, NO2, HCHO, and temperature readings. If you only see particles and temperature swipe the tile carousel to find the VOC and HCHO readings. Also check the app display settings to make sure VOC and HCHO are not hidden in the sensor display preferences.
Allow 3 to 5 Days of Continuous Operation
The HCHO electrochemical sensor in the HP09 requires 3 to 5 days of continuous operation to establish its baseline calibration for your specific indoor environment. During this period readings may appear unusually high or low as the sensor adjusts. Do not interpret readings during the first 3 days as accurate. After the calibration period the HCHO readings will stabilise and accurately reflect your room formaldehyde levels.
Update MyDyson App and Purifier Firmware
Open the App Store or Google Play and update MyDyson to the latest version. Some older app versions do not display the HCHO sensor channel for the HP09. After updating open the purifier device page and check for a pending firmware update via the About Your Machine section. Installing the latest firmware ensures all sensor channels are correctly enabled and reported through the app display.
Verify Filter Installation Is Correct
Open the HP09 and inspect the filter installation. The Dyson HP09 requires a combined HEPA and activated carbon filter. The filter must be seated correctly with no gaps between the filter edge and the purifier housing. A filter that is partially unseated allows unfiltered air to bypass the HCHO sensor preventing accurate readings. Remove and reinstall the filter firmly until the housing latch clicks. Then allow the 20-minute warmup before rechecking VOC readings.
Quick Solutions
Still having issues? This is usually the deeper cause below.
If the sensor still misses events after repositioning, check whether a scheduled 'home' or 'away' mode is overriding the sensitivity setting silently.
Place the Dyson HP09 in the room where you sleep for overnight monitoring. Formaldehyde emissions from furniture and flooring are highest in warm enclosed rooms at night and the HCHO sensor provides accurate data over long monitoring periods.
This issue almost always looks more complex than it is — the majority of cases trace back to a single setting, a stale credential, or a default that shipped wrong.
- Purifier needs ~20 min warmup for the HCHO sensor
- App display mode set to show only PM2.5, not
- HCHO sensor in calibration after a new carbon filter
- Room ventilation too high, diluting VOCs before they reach
- Firmware update changed the display; app needs updating
Before you go — try one of these (they fix most cases).
Most popular upgrades chosen by Dyson Purifier Hot+Cool Formaldehyde owners.
As an Amazon Associate we earn from qualifying purchases.
Official Manufacturer Manual
Dyson provides official product documentation through their online manual rather than downloadable PDF. Access setup guides, troubleshooting steps, and product specifications for your Dyson Purifier Hot+Cool Formaldehyde.
Source: dyson.com
Need More Help? Dyson Support
Note: The contact information below connects you directly to Dyson's official customer support team, not Trunetto. They can help with warranty claims, device replacements, and advanced technical issues.
Accessories owners commonly pair with Dyson Purifier Hot+Cool Formaldehyde.
Ring Rechargeable Quick Release Battery Pack
Ring Chime Pro
SwitchBot Hub Mini Smart Remote - IR Universal Remote, Wi...
As an Amazon Associate we earn from qualifying purchases.
