Ok, Ive been looking at putting headers on my car for a while now and I don't know if its just on my car but both of my o2 sensors are on my header and i have no o2 sensors afterwards which seems kinda pointless if you ask me to have two sensors on the header and none after the cat. The whole point of the second o2 sensor is to tell your ecu that you are still running a proper mixture and that your converter is running effectively. Which also got thinking if thats the case why the hell do i need the cat for besides legal reasons but whos going to know. The car wouldnt throw a CE light theres nothing to tell that there is a cat there or not. and in my state all the emissions testing is just done by and OBDII tester saying that if you dont have a check engine light you pass. So even though i would be putting out black smoke like a 6.5L turbo disel it wouldstill pass. Anyways back to the headers.. the only thing i see is the fact that it only utliezes the second o2 sensor position and not the first. If it only used the first o2 sensor that would be fine because i just just by MIL eliminator wires and piggy back that second o2. SO how can i make this work?

 

You actually have two CATS mzdaspd304. The one in the center area of the exhaust just after the header outlet is your main cat. There is a small warm-up cat built right into the exhaust manifold between the O2 sensors. California emssion car would also add another 02 after the main cat to check its operation as well. Not having the second sensor will definitely turn on the CEL. Unplug it for a week and try it. Even if you clear the code before going in for the inspection, your readiness test for the cat converter system would either not have been completed or would have failed on first test and set a pending code. Either way the emissions place will fail you if you have either not completed all rediness tests, or you have pending or stored codes with or with CEL being on. They thought of that already.[:@]

The solution to installing a header without a cat inside is to plug a rear O2 sensor simulator into your harness in place of sensor. One of the guys in our shop eliminated his cat because it had failed and was turning on the CEL. We have no inspection at all but the CEL kept bugging him. That was over a year and a half ago and CEL has never come on since with cat or 02 related codes.
Here are a couple of links to some.

http://www.mileliminatorsonline.com/

http://www.voodooboost.com/performan.../O2_Simulator/

 

well see thats the thing i dont have a cali. emissions car. Im from St. Louis so. From what i could tell under my car i see the header and one o2 senosr is plug in the third pipe and then when it comes to the "2" section from the 4-2-1 thats where the other o2 sensor is... there is the other section beyond that its hard to tell it looks like a flex pipe with the flange that connects the the main cat. after that was a resonator it looked like then beyond that a ways was the mid muffler..what ive done has gone to a local exhaust shop and rebuilt the exhaust with the resonator gone and built 2.5inch piping all the way to the back..removed the mid muffler and put it towards the rear...

I know what you mean. i wasnt going to remove the sensor entirely they have thoes MIL eliminator wires that splice into the heater circut of the sensor. the mil wires should remove any code that you would have with the second o2 sensor or the cat. which again should come on anyway because there is no sensor in the cat or afterwards...

If I just unplugged it how it is now, and even if i clear the code it will immideatly come back on not to mention the car will run like crap lol.

So if i get this straight there is a cat in the mainfold between the third pipe and the second section?[&:]

 

Does the diagram below look like what your exhaust used to look like? That is federal emission. You can see the two O2 sensors labeled #4 and #5. The warm-up cat is located between the two. Follow where the exhasut manifold starts with four individual runners and then as you move down the exhaust manifold they all group together and there is a bulge in the manifold before become smaller in diameter again. That bulge is where the warm-up cat is located. Your car should not run any different if you unplug the sensor that is furthest from the engine. It is only used for Cat monitor. Unplugging the one closest to the engine will make it run worse as engine computer does not know how the fuel mixture to doing.

[IMG]local://upfiles/1377/9C0834F0FFE64EABBA155FCBEC6E9405.gif[/IMG]

 

Yeah its just hard to believe that there is a cat between there because there is not even like 2ft. difference between the two. See the headers I'm looking at only seem to utitlize the second o2 sensor position and not the first and the header eliminates the "warm up cat" to which we are referring to? take a look and tell me what you think... Link VVVV

http://cgi.ebay.com/ebaymotors/OBX-E...spagenameZWDVW

 

Well it looks nice.. Install the front 02 sensor (one currently closest to engine) into the header and then plug 02 simulator into the rear 02 sensor connector. Wish they would have moved the sensor mounting hole down just a little ways into the section where all of the runners join up. That set up is going to be running your entire engine fuel mixture control based on fuel mixture from the #1 cylinder only and ignoring what the rest of the engine cylinders are doing.