# 2005 Honda Accord 4 Cylinder



## mathman32 (May 7, 2012)

Hi.
My check engine light came on yesterday. The codes are P0139 (downstream oxygen censor circuit, slow response) and P0420 (catalyst system efficiency below threshold). Here is what I am thinking about doing. I want to replace the oxygen censor and I am hoping that this will fix both of codes since the oxygen censor is connected to the cat. Also, it is WAY cheaper to start here. What do you think?


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## Mort (Nov 26, 2008)

I think you're right. How long has it been since you had a tuneup? It may be beneficial to put new plugs in it at the same time.


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## D-rock (May 23, 2011)

Those oxygen sensors are pretty common to fail, start there and see what happens


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## Yoyizit (Jul 11, 2008)

Well, mathman, by the numbers, for this decision to be a toss up, 

if the O2 sensor costs 1/10th (way cheaper?) of the catalytic stuff 
then 
the sensor should be 10x (pretty common to fail?) more likely to fail,
assuming your labor is free.

Let's price out some stuff and see what kind of odds we need for this to be a toss up. The numbers will make things more black and white.
You still may be wrong but at least you had a good reason for your choice.


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## mathman32 (May 7, 2012)

Thanks for your help people. I'm going to change the 02 censor this weekend and see what happens. 

Mort - I haven't had a tune up in a while, but I think new spark plugs may be a good idea as well. :thumbsup:


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## D-rock (May 23, 2011)

factory plugs are designed to go around 110,000 miles


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## Mort (Nov 26, 2008)

Yeah, and some cars can get 7500 miles between oil changes. That doesn't mean I'd do it.


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## D-rock (May 23, 2011)

Mort said:


> Yeah, and some cars can get 7500 miles between oil changes. That doesn't mean I'd do it.



Long life spark plugs can start to get pricey, no need to spend money on something that will have no effect on anything.


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## mathman32 (May 7, 2012)

So the 02 censor has been replaced and the check engine light has disappeared (YES!!!).


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## ukrkoz (Dec 31, 2010)

mathman32 said:


> So the 02 censor has been replaced and the check engine light has disappeared (YES!!!).


and next thing to do is to pull ECM fuse out for about 15 minutes, to clear all presets off ECM. It will learn now how to drive with new O2 sensor.


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