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#4554326 - 10/25/17 04:13 PM Should O2 sensors be replaced at the same time?
ZZman Offline


Registered: 03/17/08
Posts: 5786
Loc: Michigan
Such as both upstream or both downstream? I would think one new one and an old "sluggish" one might not be good.
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#4554328 - 10/25/17 04:14 PM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
dlundblad Offline


Registered: 09/30/13
Posts: 10033
Loc: Indiana
Don't the downstream ones just monitor emissions?
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#4554332 - 10/25/17 04:19 PM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
Chris142 Offline


Registered: 06/05/03
Posts: 16820
Loc: Deplorable in apple valley, ca
Dont bother with the down stream ones
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#4554337 - 10/25/17 04:23 PM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
NoNameJoe Offline


Registered: 06/03/15
Posts: 414
Loc: New York
The downstream ones usually only monitor emissions to make sure the catalytic converter is functioning. In that case its only purpose is to determine whether to set the MIL on or not.

The upstream sensor is the one that determines whether the engine is producing the right emissions and so that's where the control occurs that affects performance and mileage.

Once the emissions are out of the engine and the first/upstream cat determines how to control the engine, the only thing left for the second/downstream sensor to do is determine whether the cat did its job on the emissions. I assume the system was designed this way since there's no point in using both sensors to control the engine. What's the engine supposed to do differently if the cat isn't working and the second sensor doesn't like the ratio? Alter the engine to accommodate a bad cat? Nope. If the emissions from the engine itself were bad, that's something it should handle, if the emissions from engine are fine but past the cat is bad, that's nothing that the engine is responsible for, the cat failed to do its job not the engine. The only thing it can do in that case is just make the MIL light up so you can have the cat (or defective downstream sensor) fixed.

So its only purpose on most if not all vehicles is to say whether the cat is good or bad and therefore turn the MIL on or off. If you're not seeing a MIL I wouldn't bother with the downstream sensor as that's all it would do so changing it won't improve your engine performance or gas mileage or anything.

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#4554347 - 10/25/17 04:37 PM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
ZZman Offline


Registered: 03/17/08
Posts: 5786
Loc: Michigan
But should both upstreams be done together to have a correct balance?

** This being on cars at or over 100,000 miles on the sensors.


Edited by ZZman (10/25/17 04:38 PM)
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#4554367 - 10/25/17 05:07 PM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
Wolf359 Offline


Registered: 04/27/12
Posts: 4752
Loc: MA
Originally Posted By: ZZman
But should both upstreams be done together to have a correct balance?

** This being on cars at or over 100,000 miles on the sensors.


Yes. If you have two upstream sensors do them both at the same time. I suppose in theory you could just do one at the same time, but if you're going to be doing them, you mind as well do them both. It's correct about what someone else said about downstream vs upstream. I changed my upstream ones at around 140-150k, never touched the downstream ones and got rid of the car with the original downstream ones with over 200k on it.

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#4554435 - 10/25/17 06:11 PM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
LeakySeals Online   content


Registered: 02/18/11
Posts: 6573
Loc: Hudson, NH
Depends on what you are trying to accomplish. Changing the rear wont change fuel trim. But on a v6 changing both fronts for fuel trim would be a good idea.
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#4554464 - 10/25/17 06:37 PM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
Merkava_4 Offline


Registered: 01/30/07
Posts: 17501
Loc: Clovis, CA
Originally Posted By: ZZman
But should both upstreams be done together to have a correct balance?


Y E S

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#4554743 - 10/26/17 12:15 AM Re: Should O2 sensors be replaced at the same time? [Re: ZZman]
andyd Online   content


Registered: 09/25/04
Posts: 7149
Loc: Marshfield , MA
Yah,change both on a V6. Cat condition hasn't direct effect on the engine except that a sick engine will kill a cat. My firsy '88 BMW passed emissions at 335K miles on its original cat.
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#4554774 - 10/26/17 01:54 AM Re: Should O2 sensors be replaced at the same time? [Re: andyd]
Wolf359 Offline


Registered: 04/27/12
Posts: 4752
Loc: MA
Originally Posted By: andyd
Yah,change both on a V6. Cat condition hasn't direct effect on the engine except that a sick engine will kill a cat. My firsy '88 BMW passed emissions at 335K miles on its original cat.


You mean both upstream ones right? As mentioned earlier, it's the upstream ones that are in the stream of things and wear out. The engine uses them to adjust the fuel mixture. The downstream ones are after the cat so if everything is working right, they don't get much abuse and can last much longer. They're not switching much and they're there mostly to tell you when the cats are bad.

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#4554985 - 10/26/17 10:06 AM Re: Should O2 sensors be replaced at the same time? [Re: Wolf359]
LeakySeals Online   content


Registered: 02/18/11
Posts: 6573
Loc: Hudson, NH
Originally Posted By: Wolf359
Originally Posted By: andyd
Yah,change both on a V6. Cat condition hasn't direct effect on the engine except that a sick engine will kill a cat. My firsy '88 BMW passed emissions at 335K miles on its original cat.


You mean both upstream ones right? As mentioned earlier, it's the upstream ones that are in the stream of things and wear out. The engine uses them to adjust the fuel mixture. The downstream ones are after the cat so if everything is working right, they don't get much abuse and can last much longer. They're not switching much and they're there mostly to tell you when the cats are bad.

And if things go south on the cat, like a P0420 or P0430 efficiency code, you can block the sensor off from the exhaust entirely, its there for an electrical connection only.
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