Can You Trust Your CO2 Monitor?
Not all CO2 monitors are created equal, and even good ones can drift over time. Verifying accuracy ensures you are making decisions based on reliable data. Here is how to test and maintain confidence in your readings.
Quick Accuracy Checks
The Outdoor Test
The simplest check is to take your monitor outside, away from roads and buildings, and let it stabilise for 15 minutes. It should read within +/- 50 ppm of the current outdoor CO2 level (approximately 420 ppm in 2026). If it reads 350 or 500 ppm outdoors, it likely needs calibration.
The Side-by-Side Test
If you have access to a second CO2 monitor (borrow one or buy a second unit), place both in the same location for 30 minutes. Readings should agree within +/- 50-75 ppm. Larger discrepancies indicate one or both need calibration.
The Breath Test
Exhale gently near (not directly onto) the sensor from about 30 cm away. The reading should spike noticeably within 60-90 seconds, then gradually return to ambient levels. If the monitor does not respond, the sensor may be faulty.
Red Flags
Your monitor may need attention if:
- Readings consistently below 400 ppm indoors — this is physically impossible in an occupied room
- Readings never change — a stuck display usually indicates a failed sensor
- Sudden jumps of 500+ ppm — could indicate interference, a loose connection, or a failing sensor
- Readings differ wildly from a second monitor — one (or both) needs calibration
eCO2 vs True CO2 Sensors
Beware of cheap monitors that use eCO2 (estimated CO2) sensors. These devices infer CO2 from VOC levels using an algorithm, and their accuracy is poor. True NDIR or photoacoustic sensors measure CO2 directly. Learn the difference in our sensor technology guide.
Maintaining Accuracy Long-Term
- Enable ABC (Automatic Baseline Calibration) if your space is unoccupied for several hours daily.
- Perform a manual outdoor calibration every 6-12 months.
- Keep the sensor clean — dust can block the optical path in NDIR sensors. Gently blow compressed air across the sensor intake.
- Update firmware — manufacturers occasionally release updates that improve sensor compensation algorithms.
For reliable monitors that hold accuracy well, see our best home monitors guide and our Aranet4 review. Use the CO2 calculator to cross-check readings against theoretical values for your room.