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Well I had the day off today so I woke up with nothing to do. I figured since my car is still getting horrible gas mileage and since my car is at 200k and I believe it still has the original O2 sensor I figured I can go head and replace it and maybe clean the throttle body while I'm at it.

We'll I went to Advance Auto Parts and paid 20 bucks for one but when I got home and looked at it I saw that it was not the rigt one, it was the four wire kind and mine is two wires. Back to the part store to return the part and to get the right one. They don't have the right one so I go to Autozone.

Autozone has the right part but is telling me that my car has two O2 sensors. I bought two but only replaced one when I got home. I can't locate the second sensor and don't think that the car has two senors.

So what is it, one or two? If two where is it?
 

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There should be 2..... One on each exhaust manifold....
 

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Are you sure about that? I have looked long and hard and I only see one. Does anyone know for sure or what? I know what I see and right now all I see is one.
 

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I guess you saw the front one. There's onr in the back too. Both O2 sensors are screwed into the exhaust pipe not the manifold.
 

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Where is the sensor you found? If it is in the Y-pipe where the two manifolds run together, it may only have one. but if there is no way it can read the other manifold with the one you found, there is another one somewhere.
 

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I'm going by a 91 Seville I used to own. It had a 4.9 which had two O2 sensors. I guess the surest way to know if your car has two O2 sensors is to go into diagnostics. It'll have separate readings for left and right O2 sensors if you have two. If you have one sensor, it'll probably just show rich or lean. With SPFI, it should have two sensors so the system can properly read the left and right bank exhaust and adjust the injectors more precisely.
 

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Discussion Starter #10
DaveSmed said:
Where is the sensor you found? If it is in the Y-pipe where the two manifolds run together, it may only have one. but if there is no way it can read the other manifold with the one you found, there is another one somewhere.
Yes that is correct. The one I found is on the back side of the motor where the two manifolds run together. I checked on the cat and the exhaust manifold on the front of the motor for a sensor but did not see one.

I'm going to say that on my 92 Deville 4.9L there is only one. Maybe on that same motor in the previous year or a newer model they changed that design.
 

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OK, checked the parts book today and Devilles with the 4.9 have only one O2 sensor. Sevilles and Eldorados have two.
 

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El Dobro said:
OK, checked the parts book today and Devilles with the 4.9 have only one O2 sensor. Sevilles and Eldorados have two.
You say 2 for the Sevilles and Eldorados. Would that be so on the 1993s as well ?
Would be good to know for future reference.
 

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Yes....
 

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Thanx Wes !
Also....Is there a way to know if they need to be changed. Would I get a trouble code or does everything compensate so that no codes come up until there totally shot. Maybe a certain time frame or mileage when thay should be changed ?
Thinkin that maybe could be the cause for some of my problems. :banana:
 

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o2 sensors tend to go out about 60k or so.... Sure, they still WORK, but their reaction time are much slower, hence lower gas mileage...... I heard one time that if you replace your o2 sensors and you get a 1mpg increase, over 60k they pay for themselves...! And the ones for the Q45 are much more expensive... Usually there is a little performance margin to this, meaning you might increase some performace...

When the o2 sensor code come up they are completely toast.....
 

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I just replaced my O2 sensor after I got the CE light. Checked the codes and went down to Autozone. They told me my 92 DTS has 2 sensors. Well I looked high and low and even where their computer said the front one was located. The only one I can find is at the Y right before the cat. Took it out using the special tool (easy!), looked pretty bad. Put in new one and light is off. Seems to run a little smoother too. :p
 

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dts92 said:
I just replaced my O2 sensor after I got the CE light. Checked the codes and went down to Autozone. They told me my 92 DTS has 2 sensors. Well I looked high and low and even where their computer said the front one was located. The only one I can find is at the Y right before the cat. Took it out using the special tool (easy!), looked pretty bad. Put in new one and light is off. Seems to run a little smoother too. :p

A 92 DTS? Is that a pseudo DTS? :p The 4.9 has two oxygen sensors in the Eldorado and Seville applications due to the exhaust crossover (crossunder?) pipe on the E/K cars going under the oil pan from the front bank and combining near the cat.

The 4.9 has only one O2 sensor in the DeVille. The Deville exhaust crosses over from the front bank across the top of the engine/transmission bellhousing area and combines into the rear exhaust manifold before the single O2 sensor. Thus, a single O2 can "see" all cylinders in the DeVille style exhaust but two are required to see all cylinders in the E/K applications.

An O2 sensor doesn't start to get "lazy" or anything after a lot of miles. Basically an O2 sensor, especially the later model "heated" variety will fundamentally run forever. The governing factor is deposits from the fuel and especially contaminants from the fuel such as excessive sulfur, silicon, etcetera. An O2 can run for 200,000 miles with no problems or get killed by contaminants in 10k if you happen to get some contaminated fuel. No real way to tell other than to watch the codes for the O2. They're pretty robust for finding an unresponsive O2 or one that is biased due to deposits.
 

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If a 4.9 is getting poor gas mileage I wouldn't suspect the O2 as immediately as an injector or, more likely, the fuel pressure regulator. The fuel pressure regulator is easy to check by simply pulling the vacuum line off the FPR while the engine is idling. Look for the presence of fuel at the vacuum nipple on the FPR - there should be none. If there's fuel leaking from the FPR vacuum nipple then the diaphragm is leaking and the FPR needs to be replaced.
 

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I have heard that you could check if an O2 sensor is good or not
by checking the cross count. But I don't know what kind of numbers
are supposed to be OK (I know higher the number is better the
condition, but don't know actual values).

Does anybody know anything about it??
 
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