2003 Lancer ES, Auto
139k Miles

The check engine light is on, I had a buddy pull the code and it's pointing to the cat. We took the reading from the rear O2 and it reading pretty high, so seems one of the cat's is failing. The question is, is the rear cat monitored? Or just the pre-cat? My understanding is there is an O2 before the first cat, and then one afterwards but before the 2nd (main) cat. I was told that the 2nd catalytic converter is not monitored by the ECU and therefore won't throw a code if it fails, but if the 2nd o2 sensor is getting bad readings (since it sits before the 2nd cat) the ECU can throw a code.

I need to replace it if this is the problem, just trying to see if anyone has run into this before.... here is a little ASCII diagram of how it goes....or at least my understanding of how it goes. The Bold represents where I think the failure is coming from, but no way to tell without me spending a bunch of money.. so I was hoping for suggestions here. Thanks.


[ENGINE]--[HEADERS]--[1st O2]--[1st CAT]--[2nd O2]--[2nd (main) CAT]--[Muffler]

 

You have two 02 sensors. The first is in the exhaust manifold and the second is in the downstream converter.

 

So the 2nd cat could be causing that 2nd o2 sensor to throw a code? Perhaps the cat isn't working to well after 140k?

 

I'm guessing the issue is your exhaust manifold.

 

I do not see how that makes any sense....? Mind explaining your guess?

 

If your second O2 sensor is getting bad readings, it's because the first cat in the exhaust mani isn't working as the ECU thinks it should.

 

The 1st cat is built into the exhaust manifold, or can you remove it separately? I'll have to take a look when I get home.

Wonder if I can go to a junk yard and pick one up....

 

you can't remove that main catalyst in bank 1 its built into the whole exhaust manifold. it says 1st cat 2nd o2 sensor. Seems like you have a bad cat converter

 

Talked to my buddy from mitsubishi - he said they are notorious for failing even before the 36k/3 limit - said you just have to replace the whole manifold since Mitsibushi hasn't made a recall on them.

Thanks for the help guys.

 

Actually, Mitsu did a recall on the cat for the '03 Lancer OZ...or to bring it in for an ECU fix as a "preemptive" measure. My cat ended up failing anyway, even after the recall service...5 hours in the waiting room later, I had a new cat under warranty. (happened to me at about 73k.

 

If you have an aftermarket free flowing exhaust you want to use a bong spacer or slug. It pushes the 02 out a little to trick the sensor. I use it on my car cause I run no cat and works great and has for 2 years.

 

remember just cause the readings are showing o2 sensors and what not doesn't mean its the cat/o2 you could be getting these CEL readings because of something else wrong with the car like I.E vacuum leaks stuff like that. Check everything out first.

 

PIcked up a cat from the junkyard, the whoel exhaust manifold, less than 25k on it, was $140, beats the $800 from dealer, if it doesn't work... that'll suck... but we'll see this weekend when I saw it out.

 

might as well get the rrm header for that much, no cel and faster car.