Sadly it occured again a few minutes later, even after uploading the most basic arduino sketch (blink) in which I added a Serial.println. It appears sometimes it works , and sometimes not !
I ended up downloading the java IDE, which I can confirm works perfectly.

There is definitely an issue with the serial monitor when using Arduino create web IDE (and not specific to mysensors debug mode). Hope this helps !

Hi there !
Thanks for this topic which is quite uncommon but useful for beginners.
I had the same issue with a Pro Mini 3.3, also tried all possible rates in the web editor monitor.
Tried to the set MY_BAUD_RATE to 57600 with no luck. Tried with 9600, not better !
Finally tried with 19200 and now it works !

Sounds a bit weird to me, maybe the board itself is bad quality... Anyway, it's working now, thanks a lot !

Hmm, tried wiht the uart lib, after removing the unsupported parts, no luck either.

Hi there,
I'm currently trying to make it work (on an arduino pro mini 3.3), but won't succeed.
I keep having the following output :

CO2 - Sending data request to sensor.
1 - 40 - 1 - 0 - 1 - 21 - 3 - 15 - 0 - END
Wrong starting byte from co2 sensor! (should be FF)
Co2 - PPM = -1
Co2 - zzzzZZZZzzzzZZZZzzzz

Sometimes the second value is different but the first one is always 1. And obviously 1 is not FF :) . However I don't understand what's wrong in there.

I have to mention I'm using the MH-Z19B, and powering it with 5V, as opposed to the 3.3v of the arduino.
I also tried with the mhz19_uart lib, but didn't go further with it as the samples where not working out of the box with the z19B. Will probably give it another try, though...