important question... getting this done today!!!

I was told that if i run a stock cat with an aftermarket cat it may not operate properly. Is this true?
If i run two aftermarket cats, proper o2 sensors, and the like. it should make the check engine light go out. (right?) if so, my timing will be back to normal automatically (like in the sense that the ecu will fix itself... right?)

If i decide "f it... go straight pipe with mil's" it should still take out the check engine light, but will it still fix the timing (a/f ratio... etc..)

this is on my 00 mustang v6
If i run a oem cat and an aftermarket cat... what problems do you forsee if any?

um, where to start.
a) why on earth would you ever want to run more cats than absolutely necessary? They are a huge restriction in the exhaust.

b) what check engine light are you getting? Have you pulled hte codes to even see if it's related to your cat? You're obd2, so autozone can pull your code for free.

c) what makes you think your timing is off?

d) unless you tell us where your o2 sensors are, we can't tell you what exhaust mods will affect them. Chances are with straight pipes you should be fine, although you might get a check engine light showing the lack of a cat, in which case you will fail emissions (if applicable in your state). If you ask the mustang guys if straight pipes cause them to throw any codes or whatever, they will be better able to answer that, then we will.

um, where to start.
a) why on earth would you ever want to run more cats than absolutely necessary? They are a huge restriction in the exhaust.

b) what check engine light are you getting? Have you pulled hte codes to even see if it's related to your cat? You're obd2, so autozone can pull your code for free.

c) what makes you think your timing is off?

d) unless you tell us where your o2 sensors are, we can't tell you what exhaust mods will affect them. Chances are with straight pipes you should be fine, although you might get a check engine light showing the lack of a cat, in which case you will fail emissions (if applicable in your state). If you ask the mustang guys if straight pipes cause them to throw any codes or whatever, they will be better able to answer that, then we will.
Very good questions... i shoulda been more specific
a. There are 2 cats on my car... or there were before the numbskull cut one out (with the 02 sensor) before i bought it. that's why the check engine light is on... there was a hole after the headers and before the 2nd cat... no piping at all... it was loud as hell. i had a buddy weld some piping to at least complete the missing pipe, however the engine light is still on because there's no o2 sensor or cat in the front of the car.
b. I have thought about pulling the code, but i'm 99% positive it has to do with nothing being there in reference to the 02 sensor, as well as my buddy checked it when he just put the piping in there (he runs a shop here locally) he said "get the cat and 02 and it'll go away"
c. I assumed the a/f ratio as well as the timing is affected when the check engine light goes on because the ecu reads it as a failure and can not properly adjust the ratings to stock settings.

help any?

How many 02 sensors you got?

1 before the cat and one after.....
or just one....

ok, that makes alot more sense. Yeah if you are missing an 02 sensor, that would explain alot. I'm working on a mustang my friend owns later today, so I can take a look at the exhaust setup, to be sure. But if I had to guess, I'd say you could run an aftermarket and a stock cat just fine. I assume that the 2 cats are in line and not one per bank. In which case I see no problems running 2 different cats.

If they are on 2 different banks, then you might have some issues with one flowing more and causing uneven exhaust flow...but that is really the only issue I could think of off the top of my head.

Once you have the necessary cats and o2 sensors in place the ecu should reset itself just fine. Unplug the negative terminal of you battery for 15 seconds. Then just let the car idle for 10 or 15 minutes and 'relearn'.

If you get a check engine light after that, take it to autozone and get the codes pulled.

How many 02 sensors you got?

1 before the cat and one after.....
or just one....
there's 2 on this un

ok, that makes alot more sense. Yeah if you are missing an 02 sensor, that would explain alot. I'm working on a mustang my friend owns later today, so I can take a look at the exhaust setup, to be sure. But if I had to guess, I'd say you could run an aftermarket and a stock cat just fine. I assume that the 2 cats are in line and not one per bank. In which case I see no problems running 2 different cats.

If they are on 2 different banks, then you might have some issues with one flowing more and causing uneven exhaust flow...but that is really the only issue I could think of off the top of my head.

Once you have the necessary cats and o2 sensors in place the ecu should reset itself just fine. Unplug the negative terminal of you battery for 15 seconds. Then just let the car idle for 10 or 15 minutes and 'relearn'.

If you get a check engine light after that, take it to autozone and get the codes pulled.
What year mustang are you working on? Since it's single exhaust (not dual) they would not be one per bank right? i was thinking they are in line, but not 100% sure ( it would be great to have a hydrolic lift....)
Thing is, I got caught up today on maintance on the J30 that i couldn't go out there... which is good because i still need to pick up another cat (any idea which one i can use?) and an 02 sensor so he won't charge me as much for the install.

btw, thanks again.

If you already have one cat on it I don't think you necessarily need another one. I would leave the one that you have left on the car and weld a pipe in place of the other one. Either that or buy the performance cat you are talking about and weld it in place of both of the originals.

If you already have one cat on it I don't think you necessarily need another one. I would leave the one that you have left on the car and weld a pipe in place of the other one. Either that or buy the performance cat you are talking about and weld it in place of both of the originals.
Well, that's what i already have done (i got him to weld a pipe in place to complete the circuit) however, there's still no 02 sensor. and unless i get a replacement cat to hold that o2 sensor, i'll have to go with MIL's, which i've been told bad things about. My birthday is wednesday and i'd really like to be able to drive the mustang without this problem.

u think this could possibly help?

http://www.americanmuscle.com/bbk-o2-extension-harnesses.html

Dammit I worked on a 96 yesterday and totally forgot to check. Sorry. Ask in the mustang forum, surely somebody could tell you.

Also if you do delete a cat, any decent exhaust shop should just be able to weld in an o2 sensor bung for ya.

Also if you do delete a cat, any decent exhaust shop should just be able to weld in an o2 sensor bung for ya.

i was thinkin that too.

Dammit I worked on a 96 yesterday and totally forgot to check. Sorry. Ask in the mustang forum, surely somebody could tell you.

Also if you do delete a cat, any decent exhaust shop should just be able to weld in an o2 sensor bung for ya.
oh no problem govert, i've already asked the mustang forum.... sadly enough i didn't get a really good reply. What's a bung?

oh no problem govert, i've already asked the mustang forum.... sadly enough i didn't get a really good reply. What's a bung?

yeah unfortunatly a lot of the good mustang guys arent around much anymore. when i joined the forums there were a lot of great guys in there.

A "bung" is the threaded hole that the O2 sensor plugs into. That is all you need. Can you look under your car and look in front of the existing cat to see if there is a wire coming from the pipe? Does your car have 2 O2 sensors. If your car has 2 O2 sensors, then leave the one before the cat and have the bung threaded in the pipe after the cat and then plug the cat in. Reset your computer and you should be good to go...