Hiii! Can somebody help me out. I am trying to get the value from NPK sensor but it is returning 255 for all these N, P and K

I'm using this code
#include <AltSoftSerial.h>

// RO to pin 8 & DI to pin 9 when using AltSoftSerial
#define RE 6
#define DE 7

const byte nitro[] = {0x01, 0x03, 0x00, 0x1e, 0x00, 0x01, 0xe4, 0x0c};
const byte phos[] = {0x01, 0x03, 0x00, 0x1f, 0x00, 0x01, 0xb5, 0xcc};
const byte pota[] = {0x01, 0x03, 0x00, 0x20, 0x00, 0x01, 0x85, 0xc0};

byte values[11];
AltSoftSerial mod;

void setup() {
  Serial.begin(9600);
  mod.begin(9600);
  pinMode(RE, OUTPUT);
  pinMode(DE, OUTPUT);

  // put RS-485 into receive mode
  digitalWrite(DE, LOW);
  digitalWrite(RE, LOW);

  delay( 1000 );
}

void loop() {
  byte val1, val2, val3;
  Serial.print("   Nitrogen: ");
  val1 = nitrogen();
  Serial.print(" = ");
  Serial.print(val1);
  Serial.println(" mg/kg");
  delay(250);

  Serial.print("Phosphorous: ");
  val2 = phosphorous();
  Serial.print(" = ");
  Serial.print(val2);
  Serial.println(" mg/kg");
  delay(250);

  Serial.print("  Potassium: ");
  val3 = potassium();
  Serial.print(" = ");
  Serial.print(val3);
  Serial.println(" mg/kg");
  Serial.println();
  Serial.println();
  delay(5000);
}

byte nitrogen() {
  // clear the receive buffer
  mod.flushInput();

  // switch RS-485 to transmit mode
  digitalWrite(DE, HIGH);
  digitalWrite(RE, HIGH);
  delay(1);

  // write out the message
  for (uint8_t i = 0; i < sizeof(nitro); i++ ) mod.write( nitro[i] );

  // wait for the transmission to complete
  mod.flush();
  
  // switch RS-485 to receive mode
  digitalWrite(DE, LOW);
  digitalWrite(RE, LOW);

  // crude delay to allow response bytes to be received!
  delay(100);

  // read in the received bytes
  for (byte i = 0; i < 7; i++) {
    values[i] = mod.read();
    Serial.print(values[i], HEX);
    Serial.print(' ');
  }
  return values[4];
}

byte phosphorous() {
  mod.flushInput();
  digitalWrite(DE, HIGH);
  digitalWrite(RE, HIGH);
  delay(1);
  for (uint8_t i = 0; i < sizeof(phos); i++ ) mod.write( phos[i] );
  mod.flush();
  digitalWrite(DE, LOW);
  digitalWrite(RE, LOW);

  delay(100);
  for (byte i = 0; i < 7; i++) {
    values[i] = mod.read();
    Serial.print(values[i], HEX);
    Serial.print(' ');
  }
  return values[4];
}

byte potassium() {
  mod.flushInput();
  digitalWrite(DE, HIGH);
  digitalWrite(RE, HIGH);
  delay(1);
  for (uint8_t i = 0; i < sizeof(pota); i++ ) mod.write( pota[i] );
  mod.flush();
  digitalWrite(DE, LOW);
  digitalWrite(RE, LOW);

  delay(100);
  for (byte i = 0; i < 7; i++) {
    values[i] = mod.read();
    Serial.print(values[i], HEX);
    Serial.print(' ');
  }
  return values[4];
}```

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