Only can pair to ELM327, but can't establish the connection to transfer data

[hibried]

Hello, Elton. I wonder if you're still active, but I need help. So here is the problem. I'm able to pair with my ELM327, but somehow can't keep the connection established after I enter the code (1234). Here's my code:

public class MainActivity extends AppCompatActivity {

public static final int REQUEST_ACCESS_COARSE_LOCATION = 1;
public static final int REQUEST_ACCESS_FINE_LOCATION = 2;
public static final int REQUEST_ENABLE_BLUETOOTH = 11;

private ListView listview;
private BluetoothAdapter bluetoothAdapter;
private ArrayAdapter<String> listAdapter;
private List<String> device_name = new ArrayList<>();
private List<String> device_address = new ArrayList<>();
private BluetoothSocket cnt_socket;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // we get bluetooth adapter
    bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
    listview = findViewById(R.id.listview);

    // we create a simple array adapter to display devices detected
    listAdapter = new ArrayAdapter<>(this, R.layout.list_style);
    listview.setAdapter(listAdapter);

    // pairing
    listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            bluetoothAdapter.cancelDiscovery();
            startThread(position);
        }
    });

    // we check bluetooth state
    checkBluetoothState();

    // we start scanning
    scan();

    //we check permission a start of the app
    checkCoarseLocationPermission();
    checkFineLocationPermission();
}

public void scan() {
    if (bluetoothAdapter != null && bluetoothAdapter.isEnabled()) {
        // we check if coarse location must be asked
        if (checkCoarseLocationPermission()) {
            listAdapter.clear();
            device_name.clear();
            device_address.clear();
            bluetoothAdapter.startDiscovery();
        }
    } else {
        checkBluetoothState();
    }
}

public void startThread(int position) {
    BluetoothConnect btConnect = new BluetoothConnect(position);
    btConnect.start();
    Toast.makeText(this, "Connecting ...", Toast.LENGTH_SHORT).show();
}

class BluetoothConnect extends Thread {
    int position;

    BluetoothConnect(int position) {
        this.position = position;
    }

    @Override
    public void run() {
        BluetoothAdapter btAdapter = BluetoothAdapter.getDefaultAdapter();

        BluetoothDevice device = btAdapter.getRemoteDevice(device_address.get(position));

        UUID uuid = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");

        BluetoothSocket socket = null;
        try {
            socket = device.createInsecureRfcommSocketToServiceRecord(uuid);
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            socket.connect();
            cnt_socket = socket;
        } catch (IOException e) {
            e.printStackTrace();
        }

        device.createBond();
    }
}

@Override
protected void onResume() {
    super.onResume();
    // we register a dedicated receiver for some Bluetooth actions
    registerReceiver(devicesFoundReceiver, new IntentFilter(BluetoothDevice.ACTION_FOUND));
    registerReceiver(devicesFoundReceiver, new IntentFilter(BluetoothAdapter.ACTION_CONNECTION_STATE_CHANGED));
    registerReceiver(devicesFoundReceiver, new IntentFilter(BluetoothAdapter.ACTION_DISCOVERY_STARTED));
    registerReceiver(devicesFoundReceiver, new IntentFilter(BluetoothAdapter.ACTION_DISCOVERY_FINISHED));
}

protected void onPause() {
    super.onPause();
    unregisterReceiver(devicesFoundReceiver);
}

private boolean checkCoarseLocationPermission() {
    if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION)
            != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_COARSE_LOCATION},
                REQUEST_ACCESS_COARSE_LOCATION);
        return false;
    } else {
        return true;
    }
}

private boolean checkFineLocationPermission() {
    if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION)
            != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION},
                REQUEST_ACCESS_FINE_LOCATION);
        return false;
    } else {
        return true;
    }
}

private void checkBluetoothState() {
    if (bluetoothAdapter == null) {
        Toast.makeText(this, "Bluetooth is not supported on your device!", Toast.LENGTH_SHORT).show();
    } else {
        if (!bluetoothAdapter.isEnabled()) {
            Toast.makeText(this, "You need to enable Bluetooth", Toast.LENGTH_SHORT).show();
            Intent enableIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableIntent, REQUEST_ENABLE_BLUETOOTH);
        }
    }
}

@Override
protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == REQUEST_ENABLE_BLUETOOTH) {
        checkBluetoothState();
    }
}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);

    switch (requestCode) {
        case REQUEST_ACCESS_COARSE_LOCATION:
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                Toast.makeText(this, "Access coarse location allowed. You can scan Bluetooth devices", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(this, "Access coarse location forbidden. You can't scan Bluetooth devices", Toast.LENGTH_SHORT).show();
            }
            break;
    }
}

@Override
public void onBackPressed() {
    super.onBackPressed();

    Intent intent = new Intent(this, MainActivity.class);
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    startActivity(intent);
}

// we need to implement our receiver to get devices detected
private final BroadcastReceiver devicesFoundReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();

        if (BluetoothDevice.ACTION_FOUND.equals(action)) {
            BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
            if (device.getName() != null && !device_address.contains(device.getAddress())) {
                listAdapter.add(device.getName() + "\n" + device.getAddress());
                listAdapter.notifyDataSetChanged();
                device_name.add(device.getName());
                device_address.add(device.getAddress());
            }
        } else if (BluetoothAdapter.ACTION_CONNECTION_STATE_CHANGED.equals(action)) {
            try {
                new EchoOffCommand().run(cnt_socket.getInputStream(), cnt_socket.getOutputStream());
                new LineFeedOffCommand().run(cnt_socket.getInputStream(), cnt_socket.getOutputStream());
                new TimeoutCommand(125).run(cnt_socket.getInputStream(), cnt_socket.getOutputStream());
                new SelectProtocolCommand(ObdProtocols.AUTO).run(cnt_socket.getInputStream(), cnt_socket.getOutputStream());
                new AmbientAirTemperatureCommand().run(cnt_socket.getInputStream(), cnt_socket.getOutputStream());
            } catch (Exception e) {
                e.printStackTrace();
            }
        } else if (BluetoothAdapter.ACTION_DISCOVERY_FINISHED.equals(action)) {

        } else if (BluetoothAdapter.ACTION_DISCOVERY_STARTED.equals(action)) {
            Toast.makeText(context, "Showing device(s), please wait ...", Toast.LENGTH_LONG).show();
        }
    }
};

}

Would you mind to help? :(