MH-Z19B is one cheapest CO2 sensor, though not cheap as other sensors. They costs around $15 a piece. This one has been showing error code and either 0s or 400s reading. After trying with different codes and MCUs. Time to see what is inside.
from the back, we can see a circuit board in green. The sensor is enclosed by a plastic cover with golden color.
the top cover is plastic but painted with shiny gold color. Inside, it has an enclosure and a reflector
this VERY shinny golden plastic has an old bulb (hair-wired style) sticks out and a photocell showing out.
and here the circuit board with an STM chip, a bulb that pulses light on once when reading request made and a big photocell as the detector.