I have a 99 civic ex 2dr AT (d16y8). The service manual shows that the primary o2 sensor is at the header under the heat shield.

After a lot of struggling, I got the third bolt off of the heat shield and there doesn't seem to be an o2 sensor. The header also doesn't look the same as it does in the service manual. My civic is used, so maybe there have been some modifications made. Is it possible to not have a primary oxygen sensor without a CEL? I don't have a CEL, but I thought I would take a look at the o2 sensor anyways because the fuel mileage is pretty low (25-27mpg). I can't find the primary o2 sensor. Anyone know where to find it?

How do you know if the o2 sensor has gone bad? Is the CEL 100% reliable in detecting a bad o2 sensor?



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yea u can usually trust the CEL.

thats really odd....usuall the sensor is right towards the top of the header in plain view. Contac the person who had it before you, i cant tell if thats an aftermarket header or not (somone else will be able to tell).



see on those headers, right where the 4 come together theres a hole in it? Thats where the primary sensor should be.

 

that header that you have is definitly no stock looking. look under the car and see if you have a o2 sensor before and after the cat

 

It's an OEM 99 ex exhaust mani. You will have an o2 sensor just before the cat (in the downpipe), and one right in the cat like rice said

 

Yeah usually Primary 02 sensors are the "first" of the series of sensors. And the CEL is pretty reliable. You can allways check for yourself though for kicks. hook up a voltmeter to the sensor and see if it fluctuates between .200 and .800 volts. Just make sure that you don't check the wires for the heater unit if you sensors are heated. Then again, it might be a pain to get to the connector. if you are having fuel miliage issues, I would find out if the car has had allot of its major services done to it (fuel filter, air filter, fluids changed. Or if it has had allot of mods done to it that may affect it also.
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NO, on your car yes, but not on a D16Y8.

Yes it is, that's the stock manifold with the heat shield removed. If it wasn't stock it wouldn't have those stupid brackets. Trust me, I have the same one.



To the thread starter: you won't be able to see it unless you get under the car. It's right after the 2-1 merge, right before the cat. There is a short wiring harness that you can unplug, it's right there in plain view.


That gas mileage isn't terrible, BTW.

 

^+1

if it was completely bad, you'd be getting somewhere around 23-25 or less