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  #1  
Old 03-08-2008, 07:50 PM
solkoe solkoe is offline
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What does my O2 sensor do in my car?

I bought a 2000 Honda Accord recently. Nice car but a few weeks after I bought it the engine light came on. The mechanic said it was the O2 sensor. He said that it controls emissions and that if it were not fixed my car may begin to run roughly. This doesn't make sense to me because one part of his answer dealt with intake and the other with exhaust.
What does the sensor do?
And,
that engine light is one again.
I hope its the same thing since then I won't have to pay.
I hope I didn't buy a lemon.
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  #2  
Old 03-08-2008, 08:04 PM
danceswithcats danceswithcats is offline
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The short answer is that the sensor measures oxygen in the exhaust stream and provides that value to the engine mixture control computer so the rich or lean condition can be corrected to offer maximum efficiency and minimal pollution.
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  #3  
Old 03-08-2008, 08:06 PM
Rick Rick is offline
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Despite it rather misleading name, the oxygen sensor on your engine senses oxygen.
(ducks)
OK, here is the deal. The O2 sensor detects the left over oxygen in the exhaust stream and from that the computer deduces (along with the input from several other sensors) if the intake charge was rich or lean, and adjusts the mixture accordingly.
It goes like this:
Oxygen sensor senses lean ->computer enriches mixture->oxygen sensor senses rich->computer leans mixture-> oxygen sensor senses lean-. later, rinse, repeat.
Their are three basic types of oxygen sensors, if you want a design and function lesson, ask.
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  #4  
Old 03-08-2008, 08:30 PM
ManiacMan ManiacMan is offline
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How does he KNOW it is the O2 sensor?
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  #5  
Old 03-08-2008, 08:39 PM
Rick Rick is offline
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Quote:
Originally Posted by ManiacMan
How does he KNOW it is the O2 sensor?
My wild assed guess would be that the engine control module set a fault flagging the O2 sensor as being bad.
Now the technician should have done some additional fault tracing to make sure it is a O2 sensor problem and not something else that is fooling the computer to think it is an O2 sensor problem.
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  #6  
Old 03-08-2008, 08:39 PM
Bob55 Bob55 is offline
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He probably knows because they pull the codes with a fancy computer that tells them what is wrong. I've had 2 cars running now for 2 years each with the "service engine soon" lights on, both of which referring to the O2 sensor. I looked into getting it replaced and it costs like $300, or you can buy one online for $50 and do it yourself and I hear they're easy to install (just a long cable that plugs into something). I plan on doing it that way, but just haven't wanted to commit $50, not because I don't have it, but because of the principle of it. In my opinion these lights are just to force you to go to the dealer and spend money. I'm riding mine out
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  #7  
Old 03-08-2008, 08:42 PM
beowulff beowulff is offline
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Quote:
Originally Posted by Bob55
He probably knows because they pull the codes with a fancy computer that tells them what is wrong. I've had 2 cars running now for 2 years each with the "service engine soon" lights on, both of which referring to the O2 sensor. I looked into getting it replaced and it costs like $300, or you can buy one online for $50 and do it yourself and I hear they're easy to install (just a long cable that plugs into something). I plan on doing it that way, but just haven't wanted to commit $50, not because I don't have it, but because of the principle of it. In my opinion these lights are just to force you to go to the dealer and spend money. I'm riding mine out
There's a real good chance that you're wasting more money than you're saving. Do you have a history of your gas milage? If the O2 sensor is reading lean, it will cause the engine to waste gasoline.

Last edited by beowulff; 03-08-2008 at 08:42 PM.. Reason: typo
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  #8  
Old 03-08-2008, 08:50 PM
Rick Rick is offline
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Quote:
Originally Posted by Bob55
I plan on doing it that way, but just haven't wanted to commit $50, not because I don't have it, but because of the principle of it. In my opinion these lights are just to force you to go to the dealer and spend money. I'm riding mine out
Well your opinion would be wrong.
First off car makers hate emission warranties. Secondly the last time I was in Florida there were still independent garages that do repairs on O2 sensor systems. Finally your own admission that you can buy the sensor yourself is proof that no one is forcing you to go back to the dealer.
But hey if you like smog, and poor fuel mileage, knock yourself out
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  #9  
Old 03-08-2008, 11:53 PM
InLucemEdita InLucemEdita is offline
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Quote:
Originally Posted by solkoe
And,that engine light is one again. I hope its the same thing since then I won't have to pay. I hope I didn't buy a lemon.
I recommend buying a scanner. I think I paid about $25 for mine on ebay.
Simply plug it in and turn it on and you'll have the trouble code. My scanner came with a book that gives a brief explanation of the code, and you can look up more details and fixes online.

The scanner will also TURN OFF the check engine light. This doesn't fix the problem, of course, but will let you know if it was a one-time blip or a recurring problem. The O2 sensor is not very difficult to replace if I remember correctly. You may need a special socket, but they can be had at pepboys - I might be thinking of another sensor. Some wd40 will help if it is stuck on there pretty good.

Combine your scanner with a haynes or chilton manual, or better yet a factory workshop manual and you could save yourself a LOT of money over the life of a car.
1992 Firebird Formula running strong with almost 250,000 miles.
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  #10  
Old 03-09-2008, 12:13 AM
Rick Rick is offline
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Quote:
Originally Posted by InLucemEdita
This doesn't fix the problem, of course, but will let you know if it was a one-time blip or a recurring problem.
[Nitpick] A 2000 MY car will be OBD II which has the ability to turn off the check engine light all by itself if it is an intermittent problem.[/nitpick]
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  #11  
Old 03-09-2008, 12:47 PM
Bob55 Bob55 is offline
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Quote:
Originally Posted by Rick
Well your opinion would be wrong.
First off car makers hate emission warranties. Secondly the last time I was in Florida there were still independent garages that do repairs on O2 sensor systems. Finally your own admission that you can buy the sensor yourself is proof that no one is forcing you to go back to the dealer.
But hey if you like smog, and poor fuel mileage, knock yourself out
The O2 sensor that is faulty is the downstream on my main car, not upstream. So in reality it has almost no effect on mileage. And I've charted the mileage, and there is absolutely zero loss in efficiency.

I actually took the advice of both someone knowledgeable on SDMB or an auto forum years ago (when I first looked into the O2 sensor) and a family member who works with cars, and both sources said O2 sensors are not needed and you can change them at your leisure. I hear this is especially true with the downstream sensor.
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  #12  
Old 03-09-2008, 02:05 PM
Carson O'Genic Carson O'Genic is offline
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Just went through this myself recently.The car ran ragged,engine light came on. I trust my mechanic, who said it was the oxygen sensor,the one after the catalytic convertor. (Four cylinders have two,eights have four).
I had to ask about the price,as the bill seemed high.It was close to $200.00. He used original manufacturer, but said the NAPA ones are considerably cheaper.

My question for those who know,is there a difference in quality,configuration,etc.? Or are they generic ?
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  #13  
Old 03-09-2008, 02:11 PM
Rick Rick is offline
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There can be. I have seen cases where aftermarket sensors just flat don't work correctly.
It is a dilemma for a shop. The shop wants a part that works, but the lower the total bill, the better deal the customer thinks he got. However if the part fails or doesn't work at all, then it is a losing deal.
From experience a good shop learns when an aftermaket part works fine, and when they are junk.
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  #14  
Old 03-09-2008, 03:17 PM
danceswithcats danceswithcats is offline
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Quote:
Originally Posted by Bob55
The O2 sensor that is faulty is the downstream on my main car, not upstream. So in reality it has almost no effect on mileage. And I've charted the mileage, and there is absolutely zero loss in efficiency.

I actually took the advice of both someone knowledgeable on SDMB or an auto forum years ago (when I first looked into the O2 sensor) and a family member who works with cars, and both sources said O2 sensors are not needed and you can change them at your leisure. I hear this is especially true with the downstream sensor.
You're entitled to your opinion, even if it is incorrect. But, hey-why believe all of those engineers who design fuel/emission control systems?

Assuming all other engine related issues to be at factory spec, the oxy sensor could possibly be removed, without consequence. The problem is that with use, other things are subject to change. Maybe you've only incurred a loss of .2 MPG. Depending on how much you drive, that .2 could pay for a new sensor in short order, at today's fuel prices.
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  #15  
Old 03-09-2008, 07:39 PM
Carson O'Genic Carson O'Genic is offline
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Thanks Rick.
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  #16  
Old 03-09-2008, 10:06 PM
gravitycrash gravitycrash is offline
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Don't ignore the check engine light. I learned this lesson the hard way. The problem was the 0 2 sensor. I also thought it was just a pollution device, oops.

After ignoring the check engine light for about eight months the engine would not start one day. It felt like the engine had no compression. It turns out the valves had been so polluted with carbon that they would not move correctly.

1500.00 later I have learned my lesson.
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