I recently put a performance header on my 2004 Eclipse GS. Ever since I did, my Service Engine Soon light has been on. Autozone said it had something to do with my O2 Sensor but I bought a new one and still have the problem. Does anybody put an O2 simulator on their car before or have any suggestions?

Is the trouble code for the 1st Ho2 sensor or the second one? There are ways to get around this without costly simulators . Wut is the code specifically? Depending the code, with teh new header and the placement of the Ho2 sensor , with the high flow design , the sensor may now be , as ive seen before, in a too direct flow of exhaust air as was before , picking up a greater amount of heat than in specified range. Wut i have done when this is the , is gotten a spark plug non fouler , there are diffrent length ones and opened the bottom area so that the tip of the sensor can stick out thru it. The noun fouler will screw right into the O2 sensor area on the header and the O2 sensor right into the non fouler.

The code is P0421 and it is for the first 02 sensor(before the cat). Do you know what size anti fouler will fit in the O2 sensor hole? I was told that the anti fouler made for a 2004 eclipse gs will not fit into the O2 hole.

If you decide you want an 02 simulator I have one and PM me if you want it. Price will be what it costs me to ship it to you.

well use sumthin for like any sort of gm car like sumthin older V8 there are diffrent length ones tho , on teh threaded side , teh side taht will screw into the header , yu can take a grinder and ground a little off the tip/front of it or anything that will make the thru hole larger so teh tip of the O2 sensor can pertrude a little .

Altho in this case with the its hard to say , with that code , it may not be sensing enough heat , or picking up enough heat in its current position, is it located in an upper area of the header , mainlly getting just exhaust flow frum one of the cylinders , or is it near the bottom where the pipes start to connect to one at the downpipe?

The O2 sensor is right where the four pipes come into one. I think a O2 simulator is my only choice unless I mess with the ECU. I appreciate your help though.

Most likely it will be wut yu need, i think i kno wuts happening tho, The fitting that is welded into the header is a little long , not allowing the HO2 sensor to stick far enough into the down section of the header , so its not picking up the constant high stream of heat . Its causing the same effect as i stated , like when we run higher flow exhaust systems that have HO2 sensors after the catalytic converter we use the type of fitting like teh ones welded on the headers or non foulers to back the sensor out of the direct flow of air so it wont heat up as much as would having a normal high restriction cat.

Wut is happening is in yur case the sensor isnt far enough in the pipe cause of the fitting welded on the collector area to screw in the HO2 sensor , so unliek teh rear sensor that has a lower heat threshold , the pre cat one has a higher threshold but its not getting met . As stated the similator may be yur best option.