Code P0421 On 2001 Mitsubishi Galant!, Need advice on how to take care of this problem i have with my vehicle Options

[hotsheets]

I own a 2001 Mitsubishi Galant ES 4 cyl 2.4l . I have a problem on the vehicle that truly bothers me:

Code P0420 and P0421. A year ago i had the same issue, had to change the O2 sensor after catalyst, this was done and an O.E. sensor was installed after catalyst. Also the O2 sensor before catalyst was changed but the new one is a Bosch (i recently heard bad things about those).

This being, i wanted to know what would be the course of action (I was going to purchase an O.E. sensor for BANK 1 SENSOR 1 to replace the Bosch that i have currently to see if it makes the codes go away since that particular sensor most likely went bad).

I want to thank you in advance for helping me out.

Regards!

[KiT TeUnG 2549]

Well the factory sensors are Nippondenso / Denso . This code can be caused by several things. It can also be caused by the sensor it self . One thing that can cause this code is an over aged catalytic converter , if the air flow thru the converter is too high of a rate it will not allow the pre catalytic HO2 sensor to heat up to the proper heat range as desired by the PCM when the engine is cold.

A crack in the exhaust manifold / exhaust header pipe , exhaust leak pre catalytic converter , will cause small amounts of air to bypass the HO2 sensor and cause it not to heat up quickly enough during cold start, warm up modes. And lastly the HO2S itself may be aged and bad and the heating element may not be working to its full range on warm up.