Read MH-Z19 CO2 Sensor Data

mhz19.ino

// Make sure to wait the sensor to heat up first before attempting to get CO2 concentration data for the first time (approx 3 minutes preheat time).
int readCO2()
{

  byte cmd[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};
  // command to ask for data
  byte response[9]; // for answer
  
  // co2Serial is a software serial instance
  co2Serial.write(cmd, 9); //request PPM CO2

  // The serial stream can get out of sync. The response starts with 0xff, try to resync.
  while (co2Serial.available() > 0 && (unsigned char)co2Serial.peek() != 0xFF) {
    co2Serial.read();
  }

  memset(response, 0, 9);
  co2Serial.readBytes(response, 9);

  if (response[1] != 0x86)
  {
    Serial.println("Invalid response from co2 sensor!");
    return -1;
  }

  byte crc = 0;
  for (int i = 1; i < 8; i++) {
    crc += response[i];
  }
  crc = 255 - crc + 1;

  if (response[8] == crc) {
    int responseHigh = (int) response[2];
    int responseLow = (int) response[3];
    int ppm = (256 * responseHigh) + responseLow;
    return ppm;
  } else {
    Serial.println("CRC error!");
    return -1;
  }
}

@TehreemEE

I forgot that calibration is also possible by hardware, instead of software only.
When you connect the pins 'Hd' and 'Gnd' together for 7s, it should assume current level is 400 ppm.

From the manual

Sensor HD pin with low level(0V) and lasting for over 7s (under 400ppm for at least 20 minutes)

So you basically short Hd and Gnd pins for 7s outdoor.

arendst/Tasmota#3860