[esnow1]

I have an Elantra Touring that I have been modifying, I found a performance 4-2-1 header that will bolt up and that utilizes both o2 sensors, the one in the top of the stock manifold also the one in the down pipe, but it will no longer have a cat on it, so when I put the new header on, and plug in the factory o2 sensors, will I get a check engine light because there is no longer a cat on the exhaust anymore, or will it be fine since the new header will still let me use the factory o2 sensors.

[CraigB]

The CAT is after the o2 sensor so it wont affect on-board readings but removing the CAT will render your vehicle illegal and if you ever have to have your car serviced this will become an issue

Another thing to think of is the cars computer could possibly need remapping after installing extractors as well as the air flow past the sensors will change dramatically

[Johnno]

Depending on how strict your laws are for passing the emission test  you could fit a 100cpi hi flow or 200cpi sports cat, tests here in the Uk have shown the hp increase with a system which as been de-cat and one fitted with a sports cat is negligable.

Also is there not a lambda sensor on the cat? which if the cat is removed will have to be in-corporated into the new replacement pipe

I had a 100cpi race cat on my other car and it just manages and I mean just, always took a few attempts to pass the emission test, here in the UK

[Wingerdave]

I have an Elantra Touring that I have been modifying, I found a performance 4-2-1 header that will bolt up and that utilizes both o2 sensors, the one in the top of the stock manifold also the one in the down pipe, but it will no longer have a cat on it, so when I put the new header on, and plug in the factory o2 sensors, will I get a check engine light because there is no longer a cat on the exhaust anymore, or will it be fine since the new header will still let me use the factory o2 sensors.

If your not interested in emissions why not buy a seventies muscle car? Nice little 'Cuda ?     

[CraigB]

I have an Elantra Touring that I have been modifying, I found a performance 4-2-1 header that will bolt up and that utilizes both o2 sensors, the one in the top of the stock manifold also the one in the down pipe, but it will no longer have a cat on it, so when I put the new header on, and plug in the factory o2 sensors, will I get a check engine light because there is no longer a cat on the exhaust anymore, or will it be fine since the new header will still let me use the factory o2 sensors.

If your not interested in emissions why not buy a seventies muscle car? Nice little 'Cuda ?     

Because that's a hell of a lot more expensive than a set of extractors.

[Wingerdave]

I have an Elantra Touring that I have been modifying, I found a performance 4-2-1 header that will bolt up and that utilizes both o2 sensors, the one in the top of the stock manifold also the one in the down pipe, but it will no longer have a cat on it, so when I put the new header on, and plug in the factory o2 sensors, will I get a check engine light because there is no longer a cat on the exhaust anymore, or will it be fine since the new header will still let me use the factory o2 sensors.

If your not interested in emissions why not buy a seventies muscle car? Nice little 'Cuda ?     

Because that's a hell of a lot more expensive than a set of extractors.

But it would be sooooo cooooool 

[snakoil]

from my experience, the second you remove CAT or change headers on a 2000 and up car you get a CEL
Back in my 2006 Lancer ralliart I had a custom downpipe and high flow cat, i tried the plug extender solution and worked 1 day out of 2

I saw there is now an electronic device that simulates the signal, haven't tried that one yet

[Johnno]

from my experience, the second you remove CAT or change headers on a 2000 and up car you get a CEL
Back in my 2006 Lancer ralliart I had a custom downpipe and high flow cat, i tried the plug extender solution and worked 1 day out of 2

I saw there is now an electronic device that simulates the signal, haven't tried that one yet

you must have something not setup correctly.

i had pre-cat and main cat removed and just fitted a new downpipe and 100 cell race cat and I never had any problems, the new pipes had fittings tapped out in the pipe to re fit the sensors and it never threw up any codes

[i30mad]

just gut the cat, take its internals out and refit cat least you can still plug downstream sensor back in, any car in uk without a cat pre 2000 is a instant mot failure..

[CraigB]

just gut the cat, take its internals out and refit cat least you can still plug downstream sensor back in, any car in uk without a cat pre 2000 is a instant mot failure..

Once your take the core out of the catalytic converter you'll fail the mot anyway as the exhaust emissions test will fail

[cruiserfied]

And you'll get DTC's for O2 sensors.