MH-Z19B always reading 400ppm

I’ve got my MH-Z19B working (I think). I seems to always be reading 400ppm. It did initially read much higher as it warmed up. But now it reads a steady 400ppm. I’ve run both the “Zero Point” Calibration & the “Span Point”. No change. Maybe it’s reading properly but it just seems strange to me that there isn’t any change. Any input on this would be appreciated. Thanks.

Are you using the calibration properly?

Probably not as I’ve been trying to figure out how to create an environment of exactly 400ppmv and then another one of 1000-2000ppmv. It seems you need a CO2 sensor in order to calibrate your CO2 sensor. :crazy_face:

p.s. It looks like it’s not stuck on 400 anymore. It’s fluxuating a bit 401, 402, 403, 404 back to 400.

400 ppmv is the natural environmental concentration. Just put the sensor outside, while on for at least 20 minutes, then perform a zero point calibration.

A span point calibration should not be performed unless you absolutely know what you are doing. Creating an accurate CO2 concentration is incredibly difficult and you will often need a laboratory setup to do so.

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@TundraFlyer Unfortunately, unlike pH and other wet reading probes, there are no “calibration packets” for CO2 or other gasses. You need lab conditions that use CO2 meters whose annual calibration service costs more than most people’s entire setups.(…then those meters have calibrations. Eventually, it ends up at some lab tech in a white coat. It’s one of the “big fleas have little fleas on their backs to bite 'em” things. That’s likely why the Atlas CO2 probe is so much more expensive, too. Of course, it’s a final product and not a component, but it’s also factory calibrated, so that cost has to be built into the price.)

When you ran the Span Point, you calibrated the high end of the range to your room or tent air, so your scaling factor is waaaaayyyyyy off. That’s why you’re only seeing movement of a couple of ppm.

That’s why calibrating it outdoors is about as good as you can get. We don’t need NIST traceability in this use case. We’re just looking for statistically significant movement of the CO2 level.

Also, be sure your Automatic Baseline Correction is disabled in this use case. That is used in an HVAC situation when the monitored air is unoccupied like during the weekend. Your tent will never experience those conditions.

@KyleGabriel Since he ran the Span Point cal, is there a way to perform a factory reset?

The datasheet ( doesn’t mention a reset command, however a “MCU Reset” is mentioned at MH-Z19B - RevSpace but I have never attempted it and cannot verify it will work.

Just wanted to update this. The sensor was faulty. I replaced it with a new one and now I’m getting proper readings. No more stuck at 400 all the time. The old one would fluctuate for a while (anywhere between 400 & 700) only after a reboot. Then after an hour or two, get stuck at 400 & stay there. The new one is now constantly fluctuating, as you would expect, and responds to things like opening of the tent etc.

Thanks for all the help with this.

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