There are no known SQL injection vulnerabilities in the latest version of Code Igniter and the guys at EllisLabs try their best to provide fast patches for reported vulnerabilities. If you do discover a security vulnerability in the Active Record code, you should responsibly disclose it to the company developing the product. Otherwise, do not try to fix what is not broken, because you may end up ainserting vulnerabilities, rather than eliminating them.
Important note: Make sure you do not make exceptions with using Code Igniter, by constructing your own SQL syntax (especially when you use user supplied input)
$this->db->query("SELECT id FROM table WHERE column LIKE '%"."$search."'");
Or if you do, properly escape your variables using their predefined functions.