Ayuda con la funcion sumar en codeiginter[Sin respuesta]

//Sintaxis Mysql
SELECT SUM(det.subtotal) AS total,
det.id_detalle_orden,

nr.nro_orden,
veh.referencia_v,
art.referencia_Art,
det.cantidad,
det.precio,
det.subtotal,
det.iva

FROM detalle_orden AS det
INNER JOIN orden AS nr
ON nr.nro_orden = det.fk_nro_orden
INNER JOIN vehiculo AS veh
ON veh.referencia_v = det.fk_id_vehiculo
INNER JOIN articulo AS art
ON art.referencia_art = det.fk_id_articulo
WHERE det.fk_nro_orden = nr.nro_orden
GROUP BY (nro_orden);

//Modelo
public function ListarDetalle(){
$this->db->select_sum('det.subtotal total');
$this->db->select('det.id_detalle_orden,nr.nro_orden,
vh.referencia_v,
art.referencia_Art,
det.cantidad,
det.precio,
det.subtotal,
det.iva',false);


$this->db->from('detalle_orden det');
$this->db->join('orden nr','det.fk_nro_orden=nr.nro_orden');
$this->db->join('vehiculo vh','det.fk_id_vehiculo=vh.referencia_v');
$this->db->join('articulo art','det.fk_id_articulo=art.referencia_art');
$this->db->where('det.fk_nro_orden = nr.nro_orden');
$this->db->group_by('nro_orden');

$aResult = $this->db->get();

if(!$aResult->num_rows() == 1)
{
    return false;
}

return $aResult->result();

}

//Controlador
public function index() {


    $data["detalle_data"] = $this->mdetalle->ListarDetalle();
            
    $this->_load_layout("listar_detalle", $data);

}

//Vista

   <table class="table">
                    <tr>
                      <th>No.</th>
                      <th>Numero Orden</th>
                      <th >Vehiculo</th>
                      <th>Articulo</th>
                      <th>Cantidad</th>
                      <th>Precio</th>
                      <th>Iva</th>
                     <th>subtotal</th>
                     <th>Total</th>
                    
                    
                    
                    
               

                    
                      

                    </tr>
                    <?php
                    foreach($detalle_data as $row)
                    {
                     echo '
                     <tr>
                          <td>'.$row->id_detalle_orden.'</td>
                           <td>'.$row->nro_orden.'</td>
                          <td>'.$row->referencia_v.'</td>
                          <td>'.$row->nombre.'</td>
                          <td>'.$row->cantidad.'</td>
                          <td>'.$row->precio.'</td>
                          <td>'.$row->iva.'</td>
                          <td>'.$row->subtotal.'</td>
                          <td>'.$row->total.'</td>
                          
                          
                          
                         
                         

                         
                          
                     </tr>
                     ';
                    }
                    ?>
                </table>