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I just bought a DC-Sports header with no o2 sensor and i'm getting a hi flow cat. Is it bad to be running without the o2 sensor plugged in? Or do i get a o2 sim... and how do they work? How many do i need? I know this has probably been covered many times... but any help would be nice...

 
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Well I hope i say this right. But what type of car is it an LX or EX. because If i remember right the LX header has the o2 sensor in it but with the EX the o2 sensor is in the down pipe of the exhaust. and there are two o2 sensors in the exhaust one in the down pipe and one in the Cat. But I might be wrong about the o2 sensor in the header of the LX. You might be able to take it to a shop and have the hole made and so the sensor plug can screw in. Just an idea. I know with the HP racing header there are two holes to choose from and the one that is normall in the cat is behind the cat.

 
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DON'T MESS with the FIRST o2 sensor.
It's possible to run without the 2nd o2 sensor.....I am btw.

 
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uh, I have a ceramic dc sport header 4-1 EX, there not suppose to have an O2 sensor. the O2 sensor are in the hi-flow cat or the downpipe. There is two sensors one is near where ur header is and the other is near ur where ur exhaust connects to the down pipe. I have a OBX hi flow cat and it doesnt have a second O2 sensor, so my mechanic weld a nut so the O2 sensor can be screwed in. The other solution is to get an O2 stimulator without doing any welding. You will get a check engine light if u dont have the second O2 sensor or a O2 stimulator

 
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You can drive without the Second O2 sensor. I'am also driving with out my second O2 sensor. I have the HP full header with high flow cat the only problem with mine is that the wires werent long enough for the 2nd o2 sensor to reach all the way So i got it on there as much as possible with moving the wires a little bit but the engine and exhaust moved so much the wires jsut broke off the sensor. So you shouldnt have any probs without the second sensor if you upgrade your down pipe like the OBX on that 7gen was talking about.

 
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<--- 2003 ex auto. installed megan hi flow cat (downpipe with cat replacement). currently running my car with the "malfunction indicator" lamp on (its the orange engine lookalike symbol) due to bad secondary o2 sensor. first o2 sensor is very important (located on header or around), keep that no matter what. i believe the 2nd o2 sensor reads air, not sure.

a o2 mil, simulator can easily get rid of the light. but keep in mind, its not harming your engine, the computer is just reading a signal thats not so important.

 
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So as long as one is plugged in and i have an o2 sim for the second one then everything should be ok? Oh... and i have the HX... so i have 2 o2 sensors that went on the header and one down on the cat.