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Should O2 sensors be replaced at the same time? #4554326
10/25/17 04:13 PM
10/25/17 04:13 PM
Joined: Mar 2008
Posts: 5,943
Michigan
ZZman Offline OP
ZZman  Offline OP
Joined: Mar 2008
Posts: 5,943
Michigan
Such as both upstream or both downstream? I would think one new one and an old "sluggish" one might not be good.


2003 Mercury Grand Marquis. Rotella T6 Multi-Vehicle oil
2008 Sebring Hardtop convertible. Rotella T6 Multi-Vehicle oil
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Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554328
10/25/17 04:14 PM
10/25/17 04:14 PM
Joined: Sep 2013
Posts: 10,443
Indiana
dlundblad Online happy
dlundblad  Online Happy
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Indiana
Don't the downstream ones just monitor emissions?


03 Jeep WJ 4.0 200k Castrol Edge 10w40 HM Fram XG16
02 Volvo S60 2.4T 177k M1 0w40 Mahle OX149D
97 Chevy Blazer 4.3 149k Rotella T5 10w30 Supertech ST3980

Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554332
10/25/17 04:19 PM
10/25/17 04:19 PM
Joined: Jun 2003
Posts: 17,343
Deplorable in apple valley, ca
Chris142 Offline
Chris142  Offline
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Deplorable in apple valley, ca
Dont bother with the down stream ones


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Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554337
10/25/17 04:23 PM
10/25/17 04:23 PM
Joined: Jun 2015
Posts: 489
New York
NoNameJoe Offline
NoNameJoe  Offline
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Posts: 489
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.


2015 Civic LX Coupe
Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554347
10/25/17 04:37 PM
10/25/17 04:37 PM
Joined: Mar 2008
Posts: 5,943
Michigan
ZZman Offline OP
ZZman  Offline OP
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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.

Last edited by ZZman; 10/25/17 04:38 PM.

2003 Mercury Grand Marquis. Rotella T6 Multi-Vehicle oil
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Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554367
10/25/17 05:07 PM
10/25/17 05:07 PM
Joined: Apr 2012
Posts: 5,141
MA
Wolf359 Offline
Wolf359  Offline
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Posts: 5,141
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.

Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554435
10/25/17 06:11 PM
10/25/17 06:11 PM
Joined: Feb 2011
Posts: 6,718
Hudson, NH
LeakySeals Offline
LeakySeals  Offline
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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.


06 Escalade 6.0L LQ9 AWD 165k M1 5w30
03 Maxima 3.5L POS 152k ST HM 10w30
Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554464
10/25/17 06:37 PM
10/25/17 06:37 PM
Joined: Jan 2007
Posts: 17,501
Clovis, CA
Merkava_4 Offline
Merkava_4  Offline
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Posts: 17,501
Clovis, CA
Originally Posted By: ZZman
But should both upstreams be done together to have a correct balance?


Y E S

Re: Should O2 sensors be replaced at the same time? [Re: ZZman] #4554743
10/26/17 12:15 AM
10/26/17 12:15 AM
Joined: Sep 2004
Posts: 7,518
Marshfield , MA
andyd Offline
andyd  Offline
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Posts: 7,518
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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Re: Should O2 sensors be replaced at the same time? [Re: andyd] #4554774
10/26/17 01:54 AM
10/26/17 01:54 AM
Joined: Apr 2012
Posts: 5,141
MA
Wolf359 Offline
Wolf359  Offline
Joined: Apr 2012
Posts: 5,141
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.

Re: Should O2 sensors be replaced at the same time? [Re: Wolf359] #4554985
10/26/17 10:06 AM
10/26/17 10:06 AM
Joined: Feb 2011
Posts: 6,718
Hudson, NH
LeakySeals Offline
LeakySeals  Offline
Joined: Feb 2011
Posts: 6,718
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.


06 Escalade 6.0L LQ9 AWD 165k M1 5w30
03 Maxima 3.5L POS 152k ST HM 10w30

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