warm up catalyst efficiency below threshold(bank 1). any suggestions? changed cat. and rear o2 sensor.

 

Did you actually test the O2 sensor, or just replace it without testing? Test all O2 sensors. It could also be a fouled spark plug, or multiple fouled plugs.

 

Isn't there a Mitsubishi service notice about this? IIRC there is. There was a problem with it heating properly.

 

There is a Technical Service Bulletin for this code...unfortunately my car isnt covered by warranty anymore so im out of luck but youre best bet is to call a mitsu dealer by you and let them know that youre getting the p0421 and let them know theres a TSB on it and they might be able to help you...i wasnt very lucky tho =/