Got a 2010 Lancer DE 2.0 with a Injen short ram air intake and a aftermarket Tsudo Exhaust. I’ve changed the spark plugs, Mass Airflow Sensor, downstream o2 sensor and replaced my fuel tank. I also removed my front passenger seat to replace my downstream O2 sensor. I had to unplug 3 wire connections under the seat in order to pull it out. One theory I have for fixing these codes is either by replacing the other two o2 sensors (my car had three o2 sensors. One after the cat converter and two before the cat converter. Can anyone explain this?) or possibly fixing what might be wrong with the ECM. The code P0421 is either caused by a bad O2 sensor(s) or a damaged ECM which is located in the engine compartment or under one of the front seats. This code showed up after I removed my passenger front seat and replaced the downstream O2 sensor. Thoughts? Please help!!!