Honeywell dust sensor keep reporting PM2.5 at 1000 ug/m3

Hi,
I was trying to use honeywell HPMA115S0-XXX to monitor indoor PM2.5. Dust sensor is connected with Arduino mega.
Vcc-5v
RX-TX(pin 18, with 3.3v level shifter)
TX-RX(pin 19, with 3.3v level shifter)
GND-GND

#include <hpma115S0.h>

HPMA115S0 dust(Serial1);

void setup() {
  Serial.begin(9600);       
  Serial1.begin(9600);   
  dust.Init();
  dust.StartParticleMeasurement();

}

void loop() {
  unsigned int pm2_5, pm10;
    if (dust.ReadParticleMeasurement(&pm2_5, &pm10)) {
        Serial.print("PM 2.5: ");
        Serial.print(dust.GetPM2_5()); 
        Serial.println(" ug/m3" );
        delay(1000);
    }
  

}

I used the HPMA library example, and it keeps reporting 1000 ug/m3 for PM2.5. I used another sensor is around 0.1.
I’m new to Arduino. Did I make a silly rookie mistake or there is a decimal point somewhere?

Any suggestion helps.
Thank you so much!