[vulcan-works]

For people using 4.0.16 and higher this works for me

In /include/search.php
Add:

 if (!empty($data["by_productcode"])){
         $condition[] = "$sql_tbl[products].productcode LIKE '%".$data["substring"]."%'";
         if(!empty($active_modules['Product_Options'])) {
            $condition[] = "$sql_tbl[variants].productcode LIKE '%".$data["substring"]."%'";
         }
      }

AFTER:

if (!empty($data["by_fulldescr"])) {
			if($current_area == 'C' || $current_area == 'P') {
				$condition[] = "IF($sql_tbl[products_lng].full_descr IS NOT NULL AND $sql_tbl[products_lng].full_descr != '', $sql_tbl[products_lng].full_descr, $sql_tbl[products].fulldescr) LIKE '%".$data["substring"]."%'";
			} else {
				$condition[] = "$sql_tbl[products].fulldescr LIKE '%".$data["substring"]."%'";
			}
		}

and do rest as in the other posts on here