green horn from iowa.
 

hey everyone..im a newbie to the site so i apologize if im posting this question in the wrong place..but i have a 2008 eclipse w-the code po421.now i know that it is an o2 sensor and i believe its in bank 2 but when i went to order one i was asked 'front or rear' and 'upstream or downstream'..can i know which one it is by the code given or do i need to get specific readings from each individual sensor.

Welcome!

That's actually a cat code.

The problem with codes and regular people is that codes don't tell you what's actually wrong. The O2 sensors are telling you something's amiss which could be the cat, the O2 sensor or something fuel related, but at least you've got a starting point of where to look.

The problem with O2 sensors and regular people is that the computers don't see things the way we do. They deal with ones and zeroes, and they ask "which bank?" and then "sensor 1 or 2?" Left, right, up, down, none of them matter because the ECM doesn't care where the sensors are located, it's just doing math to tweak combustion. If it gets numbers it doesn't like, it throws a code and moves on with its life. Bank 1 is bank 1, whether it's one side of a V8 or it's the only bank your inline 4 has.

I generally feel safe assuming that sensor 1 is upstream and 2 is down, but which bank is which side is something I won't depend on factories either agreeing with each other or even being consistent. Especially since the code is just a starting point for diag, it's the tech's job to find out what's actually going wrong- and if it comes down to an O2 sensor that needs replacing, the tech is already there and can tell which side of the engine it's on.

Simply throwing O2s at it might wind up being cheaper than taking it to a garage. Finding a Mitsubishi forum might get you some tips on the best way to do it for yours. Good luck.

thank you for the reply..ill try the mitsub.chat.forum idea like you suggested.