Hey so slightly old (though never ending) question. P0421 is ultimately caused because of bad data or incorrect data in the PCM (Powertrain Control Module) and there is an update that can be received by a Mitsubishi Dealership to give it the correct information and prevent it from happening again.

Is anybody aware of what information was bad? Too lean, open loop, etc? What is different in the update that helps the problem from not happening again?