My experiences with the MH-Z19 NDIR CO2 sensor
Warning: I just destroyed my MH-Z19 sensor when soldering, I was changing connectors with lead free solder, iron set to 380 C. I had to heat for a little while to remove the old header. Now the sensor only gives a very low value, both on PWM and serial output.
Before destroying the device, I was getting strange values around 900 ppm when I should have been getting around 400 ppm. PWM and serial values were consistent but wrong.
My guess is that the firmware in the device for some reason thought it had a 2000 ppm range instead of 5000. Scaling the 900 ppm by 2000 / 5000 gives 360 ppm. I tried playing with the reserved "SR" pin (thinking it might mean "set range", but with no success.
Maybe I'll open my sensor up to see how it works on the inside. In any case, I'll order another sensor.