moebob

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

Vivid Racing

iTrader: (98)

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.

Green_Bandit

iTrader: (25)

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

initialg86

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 =/