MH-Z19B gets negative value

Other Hardware Sensors
1

Hi, I'm having difficulties using the MH-Z19B with my arduino nano.
I connected Vin to 5V , Gnd to Gnd, RX to D2 and TX to D3 ( I already tried switching them) and PWM to D13. There is my code, it's the example from the MHZ library :

#include <SoftwareSerial.h>
#include <MHZ.h>

// pin for pwm reading
#define CO2_IN 13

// pin for uart reading
#define MH_Z19_RX 2 
#define MH_Z19_TX 3 

MHZ co2(MH_Z19_RX, MH_Z19_TX, CO2_IN, MHZ19B);

void setup() {
 Serial.begin(9600);
 pinMode(CO2_IN, INPUT);
 delay(100);
 Serial.println("MHZ 19B");

 // enable debug to get addition information
 // co2.setDebug(true);

 if (co2.isPreHeating()) {
   Serial.print("Preheating");
   while (co2.isPreHeating()) {
     Serial.print(".");
     delay(5000);
   }
   Serial.println();
 }
}

void loop() {
 Serial.print("\n----- Time from start: ");
 Serial.print(millis() / 1000);
 Serial.println(" s");

 int ppm_uart = co2.readCO2UART();
 Serial.print("PPMuart: ");

 if (ppm_uart > 0) {
   Serial.print(ppm_uart);
 } else {
   Serial.print("n/a");
 }

 int ppm_pwm = co2.readCO2PWM();
 Serial.print(", PPMpwm: ");
 Serial.print(ppm_pwm);

 int temperature = co2.getLastTemperature();
 Serial.print(", Temperature: ");

 if (temperature > 0) {
   Serial.println(temperature);
 } else {
   Serial.println("n/a");
 }

 Serial.println("\n------------------------------");
 delay(5000);
}

I get this message :


----- Time from start: 180 s
PPMuart: n/aUnable to read value. Timeout.

Can someone explain me what I got wrong ?

2

As pin 13, (#define CO2_IN 13), has a resistor and an LED for gnd, it may affect the reading of your sensor device.
Try using another pin and report the result.

3

Thanks for your response, I tried with pin 8 but I get the same result.