My RA is at 100,700 miles and check engine light says my catalytic converter is bad. The tuning shop I trust say the parts to replace it have to be purchased from Mitsubishi and the prices are $469.00 for the Manifold and $339.00 for the center pipe. $200 for labor.

Can anyone help explain what other options I have. On another forum thread, people are saying put a high flow cat on it. Will that fix the catalytic converter? Where is a good place to learn about this?

Just buy someone's stock cat. They go for cheap and why do you need a manifold and center pipe? And $200 for undoing 4 bolts? Are they high?! You can do this in your garage over the weekend. You just need the right tools.

A high flow cat is just a better flowing catalytic converter. Yes if the cat is bad then changing it should fix the problem. I have the Boostin Performance one and I love it but Im not sure whether or not you'll need a tune just based on changing over to a high flow cat.

Ultimate Racing has some decent (reasonable quality and price) high-flow cats for the RA. They come with extended bungs to cut down on rear 02 sensor warnings.

http://www.ultimate-racing.com/xcart/home.php?cat=77

I'm gaining confidence in doing this myself. Are there any benefits to replacing with stock parts? Seems as if aftermarket is better/cheaper

make sure your cat is bad first.

causes of p0420 are

intake leaks
exhaust leaks
faulty front afr sensor
faulty rear 02 sensor

finally the cat it self

you really need to scope the front and rear o2s and check your fuel trims before you replace the cat.

after market convertors don't always work that well. are easily out gassed etc...

How do i double check my o2 sensors?

As time has gone by, I still have the same check engine light code and I'm wondering if I should get the whole catalytic converter replaced like the mechanic suggested. How can my mechanic know its not just a simple O2 sensor gone bad? Can i have the O2 sensors replaced first and see how it goes?

Did you ever have a flashing (misfire) SES light?

If not, and with your low mileage, and a tune - i suspect either the o2 sensor or the tune.

Me? I would swap out the front o2 and see if that fixes it.

Thanks Veronica, I will tell the mechanic to try swapping out the O2 sensors first.

I think you should get a high flow cat as others have suggested previously in this thread, and then get dyno-tuned or flash to one of the GST basemaps. Even if the stock cat isn't bad you'll still end up with more power in the end by getting rid of the factory cat.

^Good advice.

My RA is in the shop for unknown reasons. Thread here. But, keeping this thread on topic, I am in the market for a cat-back exhaust to replace the stock. I know I want a resonator to keep the sound dampened. I also don't want anything too large for muffler tip. This is my daily driver and I don't care to attract attention of street racing kids. I get enough attention from the fact that I drive a RA.

Here's a link below with your answer, it will be barely louder than stock so you won't need a resonator and it also maintains the stock exhaust tips. Just email this link to a custom muffler shop in your area and have them look it over, but it's pretty simple so they should be able to do it for about $100-$200(RRE charges $140 installed), and then while they are at it you can also have them fabricate a custom 3" midpipe and voila, you've got your full catback exhaust for less than $500(muffler + midpipe). If I had to do it all over again this is what I would personally do with my car.
http://roadraceengineering.com/blog/?p=1161

^This is exactly what i did and i am very pleased with the results.