I just put a DC header on my '90 Civic si, and now every time I start the car when the engine is warm the check engine light comes on and I get an O2 sensor code. If I reset the compueter it's fine until I restart it warm, and then it does it again. I used to have this header on my '89 Si and it did the same thing. I took it off and it stopped doing it. Is this common, and what's the fix? I don't think it needs an O2 sensor since taking the header off corrects the problem, but?
I'm sure someone here has run into this, so please help me figure this out.
Thanks,
Jim

 

is the o2 sensor in the same location on the header vs. the oem manifold?

 

No, it's farther away from the engine. I thought that this might be causing or contributing to the problem, but the O2 sensor appears to be in a similar location to those of many headers I've seen. I'm assuming that many people here have put a header on their car, so if it is the location I would think it would be doing it on other cars also, no?

 

hard to say. is the car obd-0? is the header meant for and obd-0 car?

 

Yes and Yes

 

is the o2 sensor good? is it plugged in tight?

 

Try putting some thread tape around the O2 sensor and re-install it. DC is notorious for making crappy headers and the bung on yours might not be threaded properly. Just my $.02

 

Yes and Yes

 

DC is notorious for making crappy headers? Who makes good ones?
I'll check the seal, but how would that make it trip a code only when starting up warm? As I said, if I start it cold it will run forever without tripping a code, but start it up warm and about a minute later the check engine light comes on.
BTW, "$.02" - Funny.

 

how do you know the o2 is working?

 

I tested it. I also don't have a problem at all if I put the stock manifold back on or run it from a cold start. There's no way the 02 sensor is bad. I even passed an emissions test last Monday with it.

 

Or you may take a good look at the header itself to see if it's cracked (i.e. leaking) before the O2 sensor. Not sure if that would cause the problem, but wouldn't hurt to check it out. And for a "good" brand of header, it's different strokes for different folks, I use a Megan Racing header on my DOHC ZC and it's still in great condition after two years. I just say the DC is crap because they are notorious for rusting, this may not be the case with yours.

 

i would guess its 1 of 2 things then since you know the sensor works. either its too far down on the dc header and the reading is wrong, or maybe you have an exhaust leak somewhere? my old accord threw the o2 cel because there was a hole in the flex pipe

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by civicandy &raquo;</TD></TR><TR><TD CLASS="quote"> either its too far down on the dc header and the reading is wrong</TD></TR></TABLE>

x2, most headers I've seen have the O2 sensor mounted right near the radiator fan.

 

which is usally only correct on obd-1 b series, that have both o2's under the heat sheild.

 

This is a 4-2-1 header. It's as far forward as it can be on the collector. If I was to move it, it would only be on a tube from one or two cylinders, and I 'm thinking that's not going to work well. If this was the cause, wouldn't everyone with this header be having the same problem? Don't get me wrong, I appreciate all the suggestions. I'm just trying to make sense of this. It seems that since I've had the EXACT same problem on two different cars onlyl when they had this header installed, that it must be related to the header, but then I don't understand why no one else has this problem. Even DC Sports themselves couldn't give me an answer. Anyone else have a DC header that also has this problem?

 

My Megan Racing header came with two possble locations. One near the top and one near the bottom. Is your's in a similar location to the top one?

 

No, it's closer to the location in the collector.

 

this is wierd u have a 90 civic si with to 2 o2 senors, funny mine only had one and the harness was never cut, i had to tack the second plug down because it aways came out. i just had the one on the top of the header. mine did the same thing warm= code but i replaced the o2 and everything was great.

but i have a 90 crx si. don't theres a diff in the wire harness.

 

You got pics of this thing?

 

i thought all obd-0's had 1 o2 sensor, except jdm. do you have 1 or 2 sensors?

 

nope dx 4-dr have them. i think dual point systems have them thats all tho. could be wrong.

 

duh! i stand corrected. my std. hatch had 2 (dual point). it had 2 now, cause it has a b16

 

So either the op doesnt have a si or went from multi port to dual point? something fishy. u should have 1 o2 senor. but please post a pic might help, i might learn something new.

 

your running a single wire non heated 02.. Since its down too far it isnt at the proper temps, you need to install a 4 wire heated 02, wire it up the same was as your old one, but youve got your signal, ground, heater and power..

Just black is signal, hook this up to the same wire going to your 02 you have now.. Then add 3 more wires.. 1 for ground, (grey or green wire) and then you have 2 white wires left, they both need to get switched ignition power 12V with Key II.

If 02 is down at bottom of header.. Needs to be heated to be at proper temps to give proper readings.