My Gravity SENO220 don't give me the good values

Hello,

I ordered a Gravity SEN0220 CO2 Sensor 0-50000 ppm 2 weeks ago. I tried it but I got a problem, the values don't go further than 5000 ppm.

I use your code to take the values and tried other codes but it's still stuck to 5000 ppm, can someone help me please ? 

The code : 

/***************************************************
* Infrared CO2 Sensor 0-50000ppm(Wide Range)
* ****************************************************
* The follow example is used to detect CO2 concentration.

* @author lg.gang([email protected])
* @version  V1.0
* @date  2016-6-6

* GNU Lesser General Public License.
* See <http://www.gnu.org/licenses/> for details.
* All above must be included in any redistribution
* ****************************************************/
#include <SoftwareSerial.h>
SoftwareSerial mySerial(10, 11); // RX, TX
unsigned char hexdata[9] = {0xFF,0x01,0x86,0x00,0x00,0x00,0x00,0x00,0x79}; //Read the gas density command /Don't change the order
void setup() {

 Serial.begin(9600);
 while (!Serial) {

 }
 mySerial.begin(9600);

}

void loop() {
  mySerial.write(hexdata,9);
  delay(500);

for(int i=0,j=0;i<9;i++)
{
 if (mySerial.available()>0)
 {
    long hi,lo,CO2;
    int ch=mySerial.read();

   if(i==2){     hi=ch;   }   //High concentration
   if(i==3){     lo=ch;   }   //Low concentration
   if(i==8) {
              CO2=hi*256+lo;  //CO2 concentration
     Serial.print("CO2 concentration: ");
     Serial.print(CO2);
     Serial.println("ppm");
              }

 }

}

}