Doing one and not the other is not a good idea. They are cheap, do them both at the same time.

 

You don't want to use a high preformance with an old tired factory cat, replace the pair.You may also want to find out what killed your cats before you go to far with this or you will just burn up a new one.
As for the 02 sensors, you get what you pay for. You can find good 02's with plugs ready to go in the mid $60 range, do it right.
Mike

 

#3 here.

I agree too.... replace both. You can find a dealer on their website. Call them and tell them they can do it for $350 if they want more. I paid $325 for both.

The O2 sensors do not come with plugs and you will not get good readings, thus negating the purpose of getting new ones.

 

Each Magnaflow (part number 94004) cat is 67 dollars, so the pair would be around 135 or 150 shipped, what are we talking about in the 350 range? I could probably find a complete Y-pipe for around 400, but installation is a bitch and I am trying to get away with spending less... I've been thinking about changing both, yes, but I do not know enough about the subject - if one of them seems to be fine, can't I just leave it untill it dies, too? The one that's dead is dead for sure, it leaks and rattles and it is dead.I feel kinda akward for removing and throwing away the good one, though... If it keep one, and change one, is this difference between them going to affect the overall performance, provided that the old one is generally good? Also, there seems to be less space on the passenger side, I wonder even if the Magnaflow cat will fit in there. So, scientifically, do they both indeed need to be replaced, or is sort of a better thing to do? I'm trying to make a decision and buy what I need ASAP, as my emissions inspection is in August, and the rattling has gotten significantly louder as of late.
Thanks for all the replies, present and future.

 

They will fit. The $325 was installed. Both. Do not just do one. even though it isn't rattling, it isn't good. You want your exhaust flow as balanced as possible.

You don't need another Y, they just cut out your old ones and throw in new ones. The reason I told you to find one of their dealers is so you could have them installed. Most midas/chains like that will not touch cats unless they are known to be bad. I got a quote from Midas, and they would do the driver side only for $450. Wouldn't touch the passenger side. Magnaflows have a lifetime guarantee, so you won't have to worry about it anymore. Make sure to get your warranty card and fill it out.

Buy the cat from the local dealer and let them install it.

 

I second all that, my magnaflows work well too.

 

OK guys, you've talked me into this - I'm going to get two Magnaflow 940004s from www.completeexhaust.comor some other such place on line, looking for a brick-and-mortar dealer is to much of a pita. My local shop will install them probably for about $250 or so in labor... So the whole fix will cost under $400. As for my O2 sensors, I bought the original expensive ones about two years ago. Any chance these will still work? That would be awesome if they did. Can anyone recomend any specific brand/place to get good aftermarket or even original O2 sensors without paying an arm and a leg? Thanks again.

 

It is easy to find a dealer. go to http://www.magnaflow.com/03dealer.aspand enter your zip. Call one of the places and get a quote. You might save enough to buy a new O2.

I am not following you on your O2 sensors. They typically start to lose their effectiveness aroung 70K. If you have ones from 2 years ago that haven't been installed, they are ok.

 

The old (not used) O2 sensors should have no problems, fortunately they don't expire. I think DiscoMikeknows a good source for cheap but good O2 sensors.

 

OK, you guys misunderstood me. Two years ago I launched operation "VA emissions inspection freedom." So those new and costly O2 sensors were installed at that point, while my left-side cat was already going bad, but not as bad as it is today. This helped me to get my check engine sign off and in factthemission was accomplished. In most of urban/suburban Virginia,unlike in some rural counties, emissions inspection needs to be passed every two years.So I got about a month to figure all this stuff out. My question was, after two years of use with a bad cat, are my O2 sensor still alive, or are they dead? Disco Mike says, there is a good chance that they will still work, or at least most of them. We'll see.