CO2 Sensors MH-Z19B FALSE

We are seeing, in recent times, an increasing number of MH-Z19B sensors more counterfeit than a 3-euro coin. These are imitations low qualitywhich are being sold on AliExpress as well as on Amazon and other retailers.

In this article I will try to shed some light on these fake sensors. Some tips for to avoid buying it, or to identify if the sensor we have received is counterfeit (or copy).


At the moment there are many Makers all over the world creating CO2 meters based on Arduino, ESP8266, ESP32 and the like, for try to help reduce the transmission of COVID through the air, especially in schools, colleges, institutes and universities.

These Makers do not suspect that the MH-Z19B CO2 sensors they are using (this is by far the most widespread model among amateurs) give the same results as the MH-Z19B sensors. worthless reading. It is important that everyone knows about it.

I consider the problem of fake MH-Z19B sensors to be important enough to ask for your help.

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What is the downside of a fake sensor?

The biggest problem of receiving a counterfeit sensor is not only the already unpleasant fact of being sold one thing for another, but also that we cannot be trusted nothing of that sensor.

The underlying problem is that most of these sensors measure false, they measure. And because the sensor gives measurements, and most people only have one sensor, cannot compare your measurements with others, and thus know if you are measuring correctly.

Fortunately or unfortunately (I mean "luck" because it has allowed me to write this article), I received one of these fake sensors a few days ago. and, as I have several CO2 sensors, it has enabled me to make some comparisons that I will share with you in this article.

Of course, if a normal user receives his sensor, switches it on and sees a CO2 concentration of, for example, 1500 ppm, he thinks (logically) that his sensor is measuring correctly. But if, as is my case, when I switch it on and see 1500 ppm I have two other sensors next to me, I trustgiving a measure of 2600 ppm The situation has changed.

In addition to this, many people think that a measurement of 1500 ppm or 2000 ppm is already too high, and this it is not so. Where I work in my house, which is a normal, normal sized room, as it could be in any house, it is not unusual to reach concentrations of 2500 and 3000 bpm if I stay for many hours at a time with the door and window closed (even if I am alone).

In the following graphic you can compare the measurements of three sensors: The two above are a Original MH-Z19B (in orange) and a Senseair S8 LP (in green). The bottom line (in blue), with much lower measurements is a MH-Z19B dummy (with the black plate). All of them were calibrated by me at the same time, outdoors, after having been there for an hour to stabilise.

Comparison of original MH-Z19B vs Senseair S8 LP vs fake MH-Z19B (copy)


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How do I know if the sensor I have is a fake?

Although there is nothing proven to 100%, and I could be wrong, I'm going to go out on a limb here and tell you how you can tell if your sensor is a fake or not quite safely.

MH-Z19B dummy (copy, black plate)

Summarised it would look like this (until I see with my own eyes tests of other sensors):

Rule 1: If your sensor plate is black and your sensor has a smooth case, your sensor is a fake.

Rule 2: In all other cases, rule 1 applies.

I am going to order this list more certain that it is false than less certain that it is false.:

Sensor MH-Z19B with black printed circuit board

Everything I know so far leads me to believe that you have a false sensor with 99.999% security..

All the sensors with a black plate that I have seen so far (or from which I have received a reference) have the box completely smoothwithout the raised contours of the original sensors.

They also have a box of a slightly more "yellowish"(although it is difficult to know which one it is if you don't have another one to compare).

In short, don't complicate things, in my experience, sí has a black plate is fake.


If you have received one of these sensors, be careful before plugging it in because, as far as I know so far, they all come with the wrong cable and if you connect it by relying on the wires (red to positive, black to negative) it will probably break down (apart from not working, of course). In the video you can see how the cable should look like, so that you can connect it without risk of damage.

MH-Z19B sensor with completely smooth housing

MH-Z19B dummy (copy, plain case)

As far as I know so far, the MH-Z19B sensors with plain case (without the embossed contours of the original sensors) are false.

As with other variations. I have only been able to test sensors with a completely smooth case with a black plate.

If you have a sensor with the plain box and green platePlease send me a photograph to document it.

Sensor MH-Z19C

MH-Z19C original (textured box)

In the case of the MH-Z19C, which are still fairly new, they appear to be original.

The photographs officers on the manufacturer's website shows a sensor with a textured boxThe box has some curves, but in the pictures of some AliExpress sellers, the box is smooth (although those curves remain).

However, the problem with this sensor is that it is very sensitive about food and can be problematic if you don't have a measuring instrument to make sure you are feeding it correctly. You can find out more about this problem here.

For the time being, the MH-Z19C sensor I do not recommend its purchase (at least for the "Home CO2 meter with Wifi"of this blog), unless you have a multimeter and understand the problem and how to solve it, as described in here.

Other sensors MH-Z19B

Although the pictures of AliExpress sellers show a whole host of varieties and variations, the reality is that I have only been able to verify the existence of the previous ones.

I will be adding to this article with new variants as they appear.

If you have a different sensor, tell us about it in the comments.

How do I avoid buying a fake sensor?

Although not foolproof, there are a few tips that can help you reduce the chance of receiving a fake sensor.

  • If the price is too good, the sensor is probably not that good. At the moment, such a sensor should cost you between 18 and 22 euros (they have gone up a bit lately, I guess as a result of high demand).
  • You will have a better chance that the sensor is original if you see that customers have posted pictures, when making their reviews, and in those pictures you can see that they have the green plate and embossing on the case.
  • Check that the reviews are good and that the customers do not say that the sensor is fake (not all reviews can always be five stars, and this is normal). If they say that the sensor is fake, it is better if they accompany it with a photograph (there are people who have had problems with their project and blame it on the sensor being fake or faulty, and it doesn't have to be that).
  • Make sure the seller has some seniority, and that their overall rating is good. I usually assume that an overall rating above 97% is good enough to give me confidence.

How do I return an MH-Z19B sensor bought on AliExpress?

If you have received a fake sensor, you will have to open a dispute on AliExpress to get your money back.

There are two reasons why you can ask for your money back:

  • The sensor is false
  • The sensor does not work correctly

My suggestion, because It has worked well for meis that you select as a reason that the sensor is not working properly, but that you make it very clear in the description that, in additionthe sensor is not working properly.

Depending on your particular case, you will have to use your own weapons. I used the comparison with two other sensors I had previously.

The message I used to open the dispute is as follows:

"Measurements are unusable This is reporting a CO2 concentration 1266 PPM. Next to it a real MH-Z19B and a Senseair S8 LP are reading 2419 PPM and 2389 PPM respectively. Attached photos of received product (black PCB, RoHS is missing, Pins are labelled differently and a different physical connection to the one offered). Attached is a video with the full unboxing and explanation. More information and explanation here:"

Along with the text, I have sent the following pictures of the sensor received:

In addition, I have sent a video of the unpacking of the product, in which you can clearly see the shipping label (this is very important for AliExpressHere you will see it with that part blurred for privacy) and the explanation of the comparison.

At less than 24 hours I had a reply from AliExpress, agreeing with me and telling me that I would get the money back on the same card with which I had paid for it. and that did not need to send the product back.

But beware, I was not given a reason why the product was fake. (which was what I had claimed), but because the product was not working properly.

This means that, if you cannot prove that the sensor is malfunctioning (as did I, who had two other sensors to compare with and the means to document it in a video). maybe it's not so easy to be proved right. This is something you will have to rate in order to steer your dispute in one direction or the other.

The video I have sent is, at its core, practically just like the video belowwithout the central explanation of what a fake sensor is, and very summarised (1:30 of unboxing and 3:45 of measurements and comparisons and with a basic, much simpler editing). I also took the time to subtitle the whole video in English.

How do I return an MH-Z19B sensor purchased on Amazon?

Yes, buying from Amazon is more expensive than buying directly from China, but, in addition to the fact that it is usually more expensive to buy from Amazon, it is also more expensive to buy from China. you receive it much soonerhas the great advantage that returning a product is extremely easy.

In this case, as I see it, you have three options:

  1. Return it to because it iswithout further explanation
  2. Return it to because it is false
  3. Return it to because it doesn't work

The difference between the first option and the other two is that, in the first case, you have to pay the return costs yourself.The other two cases will be free of charge for you.

On Amazon you can return (at your own expense for return shipping) of any item purchased for a period of 30 daysunless otherwise stated.

In cases where the return is due to the fact that the product does not work, is not as advertisedetc. you do not have to pay return postage.

This means that it is better to try to return the product. claiming that it is a fake and does not work properly and, only in the event that they do not agree with you, return it because ifeven if you have to pay the shipping costs yourself.

The return shipping costs, in cases where you have to pay for them yourself, are 2,99 if you hand it in at a post office or a Celeritas office or a 5,50 € if you ask for home collection.

In any case, if you bought the sensor on Amazon, you are guaranteed to get a refund.

MH-Z19B dummy sensor on video, full explanation, tests and data

In this video you can find additional information about the MH-Z19B dummy sensors with explanation, measurements, graphs, comparisons, etc.


I consider the problem of fake MH-Z19B sensors to be important enough to ask for your help.

Please share this page to spread the word.

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13 thoughts on “Sensores de CO2 MH-Z19B FALSOS”

  1. Good, as I said in the original post, AliExpress has given me a 19C (given because the guy cancelled so many times fake shipments that when he finally sent it, I was able to win the dispute for the last cancellation).

    My point. It's exactly the same as the one in winsen's photo, with a knurled top and only one diffusion window. Measuring measures, but I have no idea if it is accurate or not. I have mounted it in the living room and it does indeed go up when we are in the room, if you blow on it it goes up like crazy. But then if we go out it goes down on its own. I've put it down to the fact that we have a chimney and as the Monse flue can be closed, clean air continuously enters through it. But what do I know.

    Is there a way to indirectly measure accuracy?

    The temperature is more or less OK, at most one degree difference with the fetén sensor I have in my living room.

    The only electronic difference is that I have to power it with just 5v, not the esp32. Otherwise...

    You have here the sensor measurements, if you want to take a look:

    I am waiting for the display to be mounted in a box. I want it to manage the opening of the window at work. I'm dying of cold. Then maybe I'll buy another sensor. And I'll mount one at home.

    • Hello Jesus.

      That looks very good.

      In my house the measurements would be too low for the living room, with several people in it, but they may be correct in yours, especially taking into account what you say about the chimney flue always being open.

      These are the CO2 values in my living room on the day with the lowest measurements I have found for the last seven days:

      I'm looking at some homemade ways to measure accuracy (rather than measuring the accuracy of whether it measures, say, with an accurate 5%, it would be to see if it's measuring 1000 ppm when it should be 1200 or 1300 ppm (relatively large deviations). When I test, and if it works, I will prepare a video and instructions so that you can all do it. The idea is, above all, to be able to rule out sensors that are not measuring well, rather than to check the accuracy of the sensor.

      I hope that in the not too distant future it will be easier to check our sensors. When sensors are more widespread in bars, restaurants, waiting rooms and so on (they say they are already starting to be seen) it will be a matter of taking our sensor to the bar with a powerbank, having a beer and checking 😉.

      • I like that beer thing.... Well, see what you can come up with.

        I just put it in a smaller enclosed room and it seems to go up..... Although sometimes it seems to hiccup. Mind you, my house has quite a few "leaks" and is poorly insulated.

        I'm going to give it a whirl too and see how to check it out. I read somewhere that someone took CO2 out of a soda and then injected it into a known volume... Let's see... With a closed box... Filled with clean external air...

  2. Hello.
    I have the 10000ppm. They have a sticker on the side and it says so.
    It is fully embossed and looks original. It has a green badge.
    The Datasheet v1.5 already includes the three models: 2mil, 5mil and 10mil ppm.
    I control it with a Rpi and python3 in provisional mode and I haven't done graphs for so long or tests with such high values that it seems to be not working properly. I think it is the original.
    The temperature is correct compared to a BME280.
    Thank you for the article.
    Best regards

    • Hello Lluis.

      Thank you very much for confirming this.

      Indeed, in the latest datasheet (the one linked in the article "The MH-Z19B Bible") it already appears. I will update the article now.

      Best regards and thank you very much.

  3. Hi, I just subscribed, very good site. I want to set up a system as you indicate in one of your tutorials, but first I need to make sure that the sensor I buy is suitable. I just went to Amazon and saw this one, with the green plate.
    It's a bit expensive, but if it's good I'd buy it, unless you can point me to a similar, cheaper one.

    • Hello Jesus.

      The link you provide is to a sensor that ships from China (note that it says it has a delivery time of a month and a half) and is also from a newly registered seller on Amazon.

      I recommend the links already posted on the blog (in the main meter tutorial). Especially, as I explain in the tutorial, the Senseair S8 which you can find here:

      Best regards.

  4. Hi Mariete. I have just discovered your website and you have made my day, I was going to make my meter with the MQ-135 gas sensor and you have saved me a lot of work.
    I would like to know your opinion on this sensor:

    [link removed] [link removed

    The seller has 99.8% positive reviews. Sensor meanwhile has 1004 sales with 339 ratings, a positive 93%. Thanks for the site and for your feedback!

    • Hello Eduardo.

      The link you have provided doesn't look bad. The problem that I have seen several times (and that I have suffered myself) is that the suppliers keep changing and they sell both original and fake sensors, so you can't trust them. I've taken the liberty of removing the link to avoid annoying users.

      I, at the moment, can only recommend two sensors: Firstly, and by far, the Senseair S8, which is a better sensor for a little more money or the MH-Z19B on the official Winsen shop page on AliExpress. Both links are on the blog.

      Best regards.

  5. Hello everyone, and thank you for your contributions (especially Mariete).
    This paragraph is an introduction. The next one is the crux. Some time ago, I stumbled upon the page "A home CO2 meter with Wifi against the coronavirus" and I left it bookmarked with the intention of getting down to work some other time. Today I have taken it up again and I find the surprise of fake meters... My experience with Aliexpress is more bad than good, so I went to see if I could cover myself from fakes with Amazon returns, but the non-Chinese sellers ask from 37'20€ for the MH-Z19B, so I discard it, and I look for the Senseair S8 and I see the same thing; 53'49€. So I go back to Aliexpress and I look for cheap alternatives to the link Mariete provides.
    The thing is that I found this: [link removed] and the price is as attractive as it is worrying... For what it's worth, I'd like to share it with you.

    • Hello Jaime.

      The sensor you have posted is not for what we want, it is an automotive specific sensor. I have taken the liberty of removing the link you have posted so that other users do not get confused and buy something that does not work for them.

      I recommend the Senseair S8 in the link I have posted, which costs 24€.

      Best regards.


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