SCD40 / SCD41 CO2 Sensor Complete Guide

The SCD40 and SCD41 are Sensirion’s small CO₂ sensors. They measure carbon dioxide in parts per million with photoacoustic NDIR (non-dispersive infrared) sensing, and they measure temperature and humidity as well. That is a different measurement from the BME280 or the SHT sensors: none of those measures CO₂. The chip is only 10.1 × 10.1 × 6.5 mm and talks I2C.

This guide covers the difference between the SCD40 and SCD41 with the real numbers, the pins, wiring, the full command set, calibration (ASC, forced recalibration, altitude and pressure), a code example that talks to the chip, and the traps that give wrong readings. The chip data comes from the Sensirion SCD4x datasheet (version 1.5, July 2023). Breakout boards differ from seller to seller, so check the labels on your own board.

SCD40 or SCD41

Both have the same pins, address and commands, so code written for one runs on the other, except for the single shot commands that only the SCD41 knows. Take the SCD41 for battery projects, for a wider range or for the best accuracy. The SCD40 is cheaper and fine for a monitor on mains power.

Source: Sensirion SCD4x datasheet, version 1.5 (July 2023).

Getting started

Specifications

CO₂, humidity and temperature

Electrical

Source: Sensirion SCD4x datasheet, version 1.5 (July 2023). The accuracy holds at 25 °C, 50 %RH and 1013 mbar with the ASC working.

Dimensions

The chip is 10.1 × 10.1 mm and 6.5 mm high, with a protection membrane on top. Breakout boards come in many sizes, so measure your own before you design a case. Leave the membrane free: air must reach it, and a conformal coating or glue blocks it.

Source: Sensirion SCD4x datasheet.

Pins

A breakout normally brings out VDD (or VIN), GND, SDA and SCL. The order differs between makers, so read the print on yours.

Source: Sensirion SCD4x datasheet, pin assignment.

Wiring

The chip draws 15 mA on average and has current peaks, so power it from a stable supply and keep the wires short. A noisy supply (more than 30 mV ripple) disturbs the sensor. An Arduino Uno at 5 V needs a board with level shifting.

Software setup

Read the sensor

#include <Wire.h>
#include <SparkFun_SCD4x_Arduino_Library.h>

SCD4x co2Sensor;

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);
  if (!co2Sensor.begin()) {
    Serial.println("Couldn't find SCD4x sensor");
    while (1) delay(10);
  }
}

void loop() {
  if (co2Sensor.readMeasurement()) {
    Serial.print("CO2 (ppm): ");
    Serial.print(co2Sensor.getCO2());
    Serial.print("  Temp (C): ");
    Serial.print(co2Sensor.getTemperature());
    Serial.print("  Humidity (%RH): ");
    Serial.println(co2Sensor.getHumidity());
  }
  delay(5000); // the sensor itself only produces a new reading every 5s
}

I did not run this sketch on a board, it follows the library example.

Talking to the chip yourself

All commands are 16 bits and every data word has an 8-bit CRC (polynomial 0x31, start value 0xFF). Start periodic measurement, wait 5 seconds, ask whether data is ready and read nine bytes: CO₂, temperature and humidity, each followed by its CRC.

uint8_t crc8(const uint8_t *d, int n) {
  uint8_t c = 0xFF;
  for (int i = 0; i < n; i++) {
    c ^= d[i];
    for (int b = 0; b < 8; b++)
      c = (c & 0x80) ? (c << 1) ^ 0x31 : (c << 1);
  }
  return c;
}

void cmd(uint16_t c) {
  Wire.beginTransmission(0x62);
  Wire.write(c >> 8); Wire.write(c & 0xFF);
  Wire.endTransmission();
}

bool ready() {
  cmd(0xE4B8);                            // get_data_ready_status
  delay(1);
  Wire.requestFrom(0x62, 3);
  uint16_t w = (Wire.read() << 8) | Wire.read(); Wire.read();
  return (w & 0x07FF) != 0;               // lower 11 bits zero = not ready
}

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);
  cmd(0x3F86); delay(500);                // stop, in case it is running
  cmd(0x21B1);                            // start_periodic_measurement
}

void loop() {
  if (ready()) {
    cmd(0xEC05); delay(1);                // read_measurement
    Wire.requestFrom(0x62, 9);
    uint8_t b[9];
    for (int i = 0; i < 9; i++) b[i] = Wire.read();
    if (crc8(b, 2) == b[2] && crc8(b + 3, 2) == b[5] && crc8(b + 6, 2) == b[8]) {
      uint16_t co2 = (b[0] << 8) | b[1];
      float t = -45.0f + 175.0f * ((b[3] << 8) | b[4]) / 65535.0f;
      float h = 100.0f * ((b[6] << 8) | b[7]) / 65535.0f;
      Serial.printf("%u ppm  %.2f C  %.1f %%RHn", co2, t, h);
    }
  }
  delay(1000);
}

This sketch has not been run on a board. The CRC routine is the same one as in the SHT sensors, where 0xBE 0xEF gives 0x92.

Commands

The three values are CO₂ [ppm] = word 0, T = −45 + 175 × word 1 / 65535 °C and RH = 100 × word 2 / 65535 %RH. Most settings can only be changed while the sensor is idle, so stop the periodic measurement first. Use persist_settings to keep a setting after a power cycle.

Source: Sensirion SCD4x datasheet, sections 3.4 to 3.10.

Calibration: ASC, altitude and pressure

Automatic self-calibration (ASC)

The ASC is on by default. It assumes the sensor sees fresh air with about 400 ppm CO₂ at least once a week, and it corrects drift from that. If the sensor never sees fresh air (a sealed room, a greenhouse with CO₂ enrichment, a bedroom that is always occupied), the ASC pulls the readings down and the values come out too low. Switch it off with 0x2416 in that case, or ventilate the room well once a week.

Forced recalibration

Put the sensor in outdoor air for several minutes, then send the forced recalibration command with the known value (about 400 to 420 ppm outside). Do this when you cannot rely on the ASC.

Altitude and pressure

CO₂ readings depend on the air pressure. Set the altitude of your site once with 0x2427, or set the ambient pressure with 0xE000 when you have a pressure sensor, for example the BME280. Setting a pressure overrides the altitude.

Temperature offset

The chip heats itself, and the board around it heats it more. The default offset is 4 °C, valid values are 0 to 20 °C. Set the offset so the temperature and humidity match a reference thermometer, once the sensor has been running in its final enclosure. It only changes the temperature and humidity, not the CO₂ value.

Source: Sensirion SCD4x datasheet, sections 3.6 and 3.7.

Getting accurate readings

• Wait for the warm-up. The first value comes after 5 seconds, and full accuracy needs a few minutes. After assembly, the accuracy is restored by the ASC or a forced recalibration within about 5 days.
• Do not breathe on it while you test. Your breath is several thousand ppm and the reading takes a while to fall.
• Mount it in the air, not in a closed box. The response time of the CO₂ reading depends on airflow around the membrane.
• Keep it away from heat sources. Voltage regulators, displays and the ESP32 itself warm the board and raise the temperature and lower the humidity readings.
• Use a stable supply. Keep the ripple under 30 mV.
• Use the SCD41 single shot mode for batteries. One measurement every 5 minutes costs 0.45 mA on average instead of 15 mA.
• Do not cover the membrane with tape, glue or coating.

Real gotchas

• The accuracy is not a flat number. It is a number of ppm plus a percentage of the reading, and it differs between SCD40 and SCD41.
• The ASC can drive the values wrong. If the sensor never sees about 400 ppm, either switch the ASC off or ventilate once a week.
• The temperature reads too high. The chip and the board heat it. Set the temperature offset (default 4 °C) or the humidity will also be wrong.
• Wrong altitude, wrong CO₂. Set the altitude or the ambient pressure, otherwise the error grows with height above sea level.
• Settings are lost without persist_settings. Write them to the EEPROM if they must survive a power cycle. The EEPROM has a limited number of writes, so do not do it in a loop.
• Stop before you configure. Most commands only work while the sensor is idle. Send stop_periodic_measurement and wait 500 ms first.
• A single 5 V supply is not always fine. The chip takes 2.4 to 5.5 V, but the I2C lines of your microcontroller must match the supply of the board.
• The SCD40 has no single shot mode. The commands 0x219D and 0x2196 only exist on the SCD41.
• Not a safety device. Do not use it as the only protection against dangerous CO₂ levels, and it is not a carbon monoxide (CO) sensor.

Compared with other air sensors

See the BME280 guide for pressure, which also feeds the SCD4x pressure compensation, and the SHT40/SHT41 guide for accurate humidity. The last row is general knowledge and not from a datasheet read for this guide.

What to build with it

• Indoor air quality monitoring: CO2 buildup is a genuine, well-studied proxy for how stale/poorly-ventilated a room is.
• Demand-controlled ventilation: only run an extractor fan or HRV when CO2 actually rises.
• Meeting-room/classroom monitors: a common, practical use case for this exact chip.
• Greenhouse CO2 supplementation control: closing the loop on CO2 enrichment for plant growth.

• A CO₂ traffic light with three LEDs: green below 800 ppm, orange up to 1200 ppm, red above.
• A HomeKit air-quality sensor on an ESP32.

Safety notes

Conclusion

The SCD40 and SCD41 are real CO₂ sensors that fit in a few square centimetres. Take the SCD41 for a wider range, better accuracy and a low-power single shot mode, and the SCD40 for an always-powered monitor. Set the altitude, the temperature offset, and decide on the ASC, then the readings can be trusted. Keep it away from heat and leave the membrane free.

Chip data is from the Sensirion SCD4x datasheet, version 1.5. The comparison row for gas sensors is general knowledge.