Interfacing of CO2 sensor module with Arduino ATMega328 through ttl

Projects Programming
12

Please youse code tags so your code looks like

#include <SoftwareSerial.h>
SoftwareSerial mySerial(4, 5); // RX, TX
byte GetData[] = {0x80,0x02,0x00,0x7E};
void setup() {
  // initialize serial:
  Serial.begin(19200);
}
 void loop()
{
  Serial.write(GetData,sizeof(GetData));   
}

Is that the complete program?
It does not use SoftwareSerial at all.
You need a delay() - perhaps delay(300); - in loop() to slow it down so it does not overwhelm the serial interface.
Are you viewing the output in the Serial Monitor?
0x7E is the only one of your values that matches a printable character - what are you expecting to see?

...R