How to tell if the O2 sensors need replacing?

TonyT

Original poster
Member
May 6, 2012
108
I'm wondering how you can tell if the O2 sensors need replacing? Anything you can do with the OBD2 and software monitoring things as you go for a highway drive?
 

Matt

Member
Dec 2, 2011
4,024
Yes there is. The forward sensor should fluctuate between, IIRC, 0.1-0.8 fairly rapidly. If it stays static, it's gotten lazy and will need to be replaced. The rearward sensor should stay pretty much static.
 

RayVoy

Member
Nov 20, 2011
939
I change mine when I change the plugs, 100k is a good time to change 'em. Performance and mileage always improves.
 

TonyT

Original poster
Member
May 6, 2012
108
Matt said:
Yes there is. The forward sensor should fluctuate between, IIRC, 0.1-0.8 fairly rapidly. If it stays static, it's gotten lazy and will need to be replaced. The rearward sensor should stay pretty much static.
You do indeed recall very correctly. The forward sensor was between 0.1 and 0.8 several times per second. Some highs and lows were higher and lower than the min & max. That is sometimes the high was 0.6 or 0.7 awhile the low was 0.2 or even 0.4 or 0.5 a few times.

A little later. As I'm going back and reviewing the logged data there are times when the front sensor changes about every one or two seconds with a bit of a sine wave or curved shape. But other times where it's changing 2 or 3 times per second.

The rear sensor stayed around 0.7 with minor fluctuations of around 0.02 of a volt. No that was on the highway. While idling the rear sensor fluctuated quite a bit.

I moved the engine RPM recording to minimize the number of lines on the graph. Next time I do this I'll add it back so I have a better idea of it's behavior at what point.
 

Chickenhawk

Member
Dec 6, 2011
781
Maybe show us the graphs if there is any way. The front sensor should fluctuate between 100 and 900 mV very rapidly. If it only changes every 1 to 2 seconds, that is too slow and may be getting lazy and due for replacement.

There are videos on Youtube with a properly functioning sensor.

The rear should be fairly solid, even at idle. If the fluctuations ever seem to be as fast as the front, either your front is really slow or your cat needs replacing.

I am putting money on the former.
 

fadyasha

Member
Dec 21, 2011
1,134
I just change them every 100k. There was a discussion on the OS once that this should be a regular maintenance
 

TonyT

Original poster
Member
May 6, 2012
108
fadyasha said:
I just change them every 100k. There was a discussion on the OS once that this should be a regular maintenance
I'm a relatively new owner so I have no idea when, or if, they were ever replaced previously. I'm getting quite tempted to replace them both and be done with it.
 

TonyT

Original poster
Member
May 6, 2012
108
Chickenhawk said:
Maybe show us the graphs if there is any way. The front sensor should fluctuate between 100 and 900 mV very rapidly. If it only changes every 1 to 2 seconds, that is too slow and may be getting lazy and due for replacement.

There are videos on Youtube with a properly functioning sensor.

The rear should be fairly solid, even at idle. If the fluctuations ever seem to be as fast as the front, either your front is really slow or your cat needs replacing.

I am putting money on the former.
For a minute or four on my highway drive it was every 1 or 2 seconds which was at about 62 mph. But later in that 15 minute highway drive it was twice a second when I was doing 74 mph.

It's a bit of a pain to scroll backwards in the graph in the software. You have to drag and drop rather than, for example, just pressing the left and right arrow keys.

I'll see if I can post an example of the two different chunks but it won't be for a day or until Tuesday or so.
 

Chickenhawk

Member
Dec 6, 2011
781
Don't bother. Change the front sensor. There is no question it is too slow.

A good rule of thumb is to change the front sensor with every plug change. AC Delco sensors seem to be the ones with the fewest problems. (If left too long, even a good sensor can seize to the manifold, making removal a real chore.)

I wouldn't bother changing the rear quite yet. They rarely fail, and even then the rear does not affect performance in any way. It is there to check the performance of the cat converter. (As long as your coolant gauge normally sits with the needle straight up or a tiny tick to the right when warm, your cat is probably just fine.)
 

TonyT

Original poster
Member
May 6, 2012
108
Chickenhawk said:
Don't bother. Change the front sensor. There is no question it is too slow.

A good rule of thumb is to change the front sensor with every plug change. AC Delco sensors seem to be the ones with the fewest problems. (If left too long, even a good sensor can seize to the manifold, making removal a real chore.)

I wouldn't bother changing the rear quite yet. They rarely fail, and even then the rear does not affect performance in any way. It is there to check the performance of the cat converter. (As long as your coolant gauge normally sits with the needle straight up or a tiny tick to the right when warm, your cat is probably just fine.)
How much are these sensors anyhow? $$$ If they're not too bad I'd just as soon do both and be done with it.

I'm relatively new to the vehicle so I have no idea when the plugs were done. I'll get them done soon too though just to be on the safe side.
 

Matt

Member
Dec 2, 2011
4,024
US$50 for the front and US$38 for the rear from Rock Auto.
 

TonyT

Original poster
Member
May 6, 2012
108
Matt said:
US$50 for the front and US$38 for the rear from Rock Auto.
What's the book time for replacing the front one and the rear?

I ask because the stealership is going to replace my fuel tank but want to charge me an hour for the fuel pump replacement so the labour will be covered for warranty purposes. I'm thinking they just want to gouge me for an hours labour. But if so then I'll tell them to put the hours labour on the fuel pump but give me a separate invoice for the O2 sensors with no labour.

Just to get their knickers in a twist. :smile:
 

Menthol

Member
Dec 8, 2011
177
TonyT said:
What's the book time for replacing the front one and the rear?

I ask because the stealership is going to replace my fuel tank but want to charge me an hour for the fuel pump replacement so the labour will be covered for warranty purposes. I'm thinking they just want to gouge me for an hours labour. But if so then I'll tell them to put the hours labour on the fuel pump but give me a separate invoice for the O2 sensors with no labour.

Just to get their knickers in a twist. :smile:
If it's warranty work (fuel pump) they have a set time they get paid to do that.
 

CaptainXL

Member
Dec 4, 2011
2,445
Alldata lists 2 hours labor to replace the fuel pump and .2 hours for 02 sensor under manufacturers warranty.
 

TonyT

Original poster
Member
May 6, 2012
108
CaptainXL said:
Alldata lists 2 hours labor to replace the fuel pump and .2 hours for 02 sensor under manufacturers warranty.
Two hours labour to replace the fuel pump which will likely be the same as the labour to replace the fuel tank as you have to drop the tank either way.

Oh, it's that easy to replace the O2 sensors. Thanks for that info.

If the stealership insists on telling me I have to pay them an hour to replace the fuel tank to get a warranty on the fuel pump I'll simply ask them for the fuel tank and pay my mechanic two hours to do it I'd sooner pay him than the out of town owner making a profit. It's the principle and probably not that much more money anyhow.

No, on second thought I'm going to get them to change the tank. They'll loosen the nuts nicely, etc, etc. Then I'll just take it to my mechanic. And have him remove the tank again to replace the fuel pump. And I'll make sure the service manager knows what I'm doing. Not that he'll care.

F*** them.
 

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