So apparently the underbody cat on my almost stock Evo8 is shot, which is what is making it rattle after 4000rpm. Also a small exhaust leak on the stock resonator, which I can weld up myself.

I don't want to go catless/testpipe since I have to do emissions next year and I have a feeling that I wouldn't pass otherwise.

Other than OEM (which seems to be around the $900 mark...) what other options do I have that would be a bolt-on affair doing it myself? If I go with one of the multitude of highflow cats, or aftermarket cats, do I need to do a tune afterwards, i.e. get a check engine because the post cat O2 sensor gets a reading it doesn't expect? What about your run-of-the-mill exhaust shop and their off the shelf aftermarket cat?

Trying to keep it cheap and easy.

Thanks!

There's a guy in the electronics section who installed a new cat and the monitor won't set. I suspicion it is a high flow cat.

Have you thought about just picking up a universal cat from autozone and having an exhaust shop weld on flanges? You could use it just to pass smog and it's much cheaper.

http://www.autozone.com/emission-con...07_460559_5320

If your location is accurate in this area emissions does not do a sniffer test. The HFC is a good bet. The non race cell HFCs don't require a tune and won't through an codes...the race HFCs (100cel or thereabouts) will throw codes when the car warms up on cold days. You can either clear them before the test or disable the rear O2 sensor with a tune. Hope that helps!

Sorry I know this thread is kinda old but can't find anything about this, on my second cat i have a downstream o2 sensor if I install high flow cat and move the o2 down will it throw a cel again thank youfor the help