First: Is the Reading Actually Wrong?
A surprisingly high CO2 reading usually means one of two things — your air really is that bad, or your monitor needs attention. Before assuming a fault, remember that closed bedrooms routinely hit 1,500-2,500 ppm overnight, and a small meeting room can pass 2,000 ppm in under an hour. High readings are often correct.
Here are the eight most common causes, roughly in the order you should check them.
1. The Room Genuinely Needs Ventilation
People are CO2 machines: each occupant exhales roughly 1 kg of CO2 per day. Two people in a closed 12 m² bedroom will push levels past 2,000 ppm by morning. Open a window for ten minutes — if the reading falls steadily toward 420-600 ppm, your monitor is working perfectly. Our ventilation guide covers low-cost fixes.
2. You're Breathing on the Sensor
Exhaled breath contains about 40,000 ppm CO2 — a hundred times room level. Standing over the monitor, or placing it on your desk right in front of you, produces dramatic spikes. Follow our placement guide: breathing height, at least 1-2 metres from people, away from windows and vents.
3. Automatic Calibration Has Drifted
Most consumer monitors use ABC (Automatic Baseline Calibration), which assumes the room sees fresh outdoor air (~420 ppm) at least once a week. In a continuously occupied home that never airs out, ABC re-anchors the baseline too high and every reading inflates. The fix is a manual calibration: put the monitor outside in the shade for 15-20 minutes and trigger the calibration routine.
4. It's Not a Real CO2 Sensor
Many cheap "air quality monitors" report eCO2 (estimated CO2) derived from a VOC sensor. These are notoriously wrong — cooking, cleaning sprays, or a glass of wine can send eCO2 to absurd values. Check whether your device has a true NDIR or photoacoustic sensor; if it lists "eCO2" or "CO2 equivalent", the number is an estimate, not a measurement.
5. Combustion Sources Indoors
Gas stoves, candles, unvented heaters, and fireplaces all emit CO2 directly. A gas hob can add several hundred ppm to a kitchen in minutes. (They can also emit carbon monoxide — a different, dangerous gas that your CO2 monitor cannot detect.)
6. New Occupants: Pets, Plants at Night, Guests
More lungs, more CO2. Dogs and cats contribute meaningfully in small rooms, and plants release CO2 in darkness — though far less than people assume. A dinner party can double your usual levels.
7. Sensor Warm-Up or Low Battery
NDIR sensors need a few minutes after power-on to stabilise, and some units misread as batteries fade. Give the monitor 10 minutes on fresh power before trusting the number.
8. The Sensor Has Failed
NDIR sensors typically last 10-15 years, but a unit that reads high even outdoors after a manual calibration is faulty. Verify against a known-good reference — our guide to verifying accuracy walks through the process.
Quick Diagnostic Flowchart
- Take it outside for 15 minutes. Reads ~400-450 ppm? The monitor is fine — your room needs air. Use the ventilation calculator to size the fix.
- Still high outdoors? Run manual calibration.
- Still high after calibration? Check for eCO2 vs true NDIR, then consider replacement — our under-$100 guide has reliable options.