MIL light illuminates as a result of a P0420 through P0439 diagnostic trouble code.
Emissions are Elevated as Indicated by P0420 - P0439 Code
These codes typically indicate increased emissions. Increased emissions may be due to improper air/fuel ratio (converter feeds gases). Other possible causes may include converter damaged due to misfires, internal contamination or physical damage (contact with road debris).
Begin by repairing/replacing any mechanical or ignition related failed component. Next inspect the exhaust system for leaks. Keep in mind that even small pin hole leaks around O2 sensors, hairline cracks in manifolds and minor gasket leaks can cause issues with proper converter operation. Next, pressure test the cooling system to make sure that the converter is not being poisoned by an internal coolant leak, and then check for proper fuel control.