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O2 Sensor Help

Hi All -

I have an 08 Lancer. My cat is bad so I’m replacing it (it’s a direct fit but doesn’t include the muffler or resonator), but I need to replace the o2 sensors as well. Everything I’ve found online says that I have 3 sensors one in the manifold, one pre-cat and one post-cat. When I looked under the car, the sensors I found are located before the cat and IN the cat. The exhaust I bought matches what I have underneath.

My question is, can anyone help me identify what o2 sensors these are so I can buy them to replace (e.g. upstream, downstream, center)


So, you have a total of two sensors, right?

Apparently there’s 3. One is in the manifold.

Go here,,2008,lancer,2.0l+l4,1438377,exhaust+&+emission,oxygen+(o2)+sensor,5132 and it identifies what sensor goes where.


Yeah I’ve been to rockauto. My problem is that I have no sensors after the cat. The final o2 spot I have is on the cat. So am I just shifting over so that the center sensor is in front of the cat and the downstream is in the cat?

Okay so I’ve been doing more looking into this, and apparently those are the only 2 sensors. My question is then, is that normal to have the rear sensor in the catalytic converter?

What was the configuration of the car before you took it apart?

If the new one doesn’t match, it’s the wrong part.

I haven’t taken anything apart. I only crawled underneath to look, and have been scouring the internet for exhaust diagrams to verify what i need to know. It looks like those are the only 2 sensors, which I’m guessing are the upstream and downstream

The O2 sensor in the center of the cat monitors the primary catalyst.

Your vehicle doesn’t monitor the secondary catalyst.



Next question then… I’m looking to buy the o2 sensors, and am seeing universal placement as well as upstream and downstream. Should I not go with the universal fit? Or are those alright to use?


This sensor can be used before the cat and after the primary catalyst.

The reason that there’s an O2 sensor before that cat is, the sensor in the manifold isn’t an O2 sensor. But instead an air/fuel ratio sensor.

And in order to measure catalyst efficiency requires two O2 sensors so both voltage outputs can be measured by the computer.



The reason they recommend replacing the oxygen sensors at the same time is that it’s difficult to tell which one is at fault, the oxygen sensors or the catalytic converter. It’s kind of like replacing the water pump when you do a timing belt job so you won’t have problems later. With that in mind, I might be tempted to use the old oxygen sensors if you can get them out (and they aren’t fused in place). If the “check engine” light never goes away or comes back, you could replace them at that time (as long as they’re not hard to get to with the cat installed).

If you decide to replace them, my recommendation would be to take them out and show them to someone at an auto parts store or Mitsubshi dealership. That way you can make sure to get the right ones. Personally, I’d stay away from universal oxygen sensors and go with OEM sensors, even if they cost more. They will have a direct plug. Universal sensors often require you to splice some wires to the old connectors, and sometimes they don’t make it easy by having the same number and colors of wires.

Don’t know how common it is, but that’s the way it was in my '07 Civic.

I think it’s been common for quite some time. My 19 year old car has two oxygen sensors, one before and one after the catalytic converter.

Exactly how did “they” diagnose the cat is bad?

Seems to me if a proper diagnosis was done, then replacing the oxygen sensors isn’t necessary

Since you want to replace everything, makes me wonder how the diagnosis was conducted . . . unless the cat is physically damaged or rusted out

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We keep getting cat replacment questions. My pet peeve, though peeve is a weak word, is that many mechanics believe, wrongly, that P0420 means the cat is bad. On my 2002 Sienna with 220,000 miles, i have had P0420 several times and in every case it was a bad sensor.

that is why db asks how it was diagnosed.

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I diagnosed based on the boards here. Had the P0420, but also had residue around my exhaust exit. Had overheating issues associated with a loss of power when overheating. Couple that with the fact that it accelerated like a slug, I found someone say on here that those point to a bad cat.

So then update on this: replaces the cat today. Stupid place I ordered from sent me the wrong part (I was missing the port for the upstream o2 sensor and the rear flare on the pipe wasn’t big enough to fit around the old exhaust pipe.

Solution: salvage the old pipe from the manifold to just after the upstream sensor/before the old cat. Cut the other end of the old pipe the appropriate length. Crimp this end of the old pipe so it fits in the flare and weld the joint. Repeated at other end (didn’t need to crimp that end, just welded it in place). Attached the mounting bracket (the only thing that lined up right), put in new sensors.

Result: I have power on acceleration again, I have no overheating issues, the welds are holding solidly on place (I’m so much better with an arc welder instead of a mig welder, but didn’t have access to an arc), and most importantly… no codes.

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I have not personally had a plugged cat, but that sure sounds exactly like a plugged cat, not an inefficient one.

If you post again on another problem, give us those details in the OP. It helps things move along faster. And, the pros here would have instantly given you a correct diagnosis/recommendation.

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Good for you for figuring out the proper work-a-round exhaust configuration and getting your Lancer back on the road with full power and no codes OP. Thanks for posting the final resolution to the problem.