Varelco 211 Report post Posted September 17, 2016 Hi Folks, Quick question for the tuners and people in the know, is the secondary O2 sensor (the one downstream of the cat) used for any control purpose by the ECU? As in fuelling and/or ignition, or does it purely exist to monitor the efficiency of the cat? I know this varies between cars. Share this post Link to post Share on other sites
