Monitor Strategy Description
WARNING: This page is about a different car, the 2006 Suzuki Verona. However, it is still accessible from the selected car via links, so may be relevant.
There is a catalyst diagnosis with separate Cat-Diag values for each exhaust gas bank. If one exhaust bank value is over calibrated threshold, this catalyst is detected as faulty. That means it one catalyst is aged over OBD limit the MIL is switched on.