Benefits of Changing O2 Sensors>?

Status
Not open for further replies.
Joined
Sep 21, 2004
Messages
502
Location
Atlanta, Ga.
Just curious...
confused.gif


I have close to 165K on a Ford Crown Vic, which I took out of service at 78K. I know for sure, since I have had the vehicle, the O2 sensors have never been changed.

Is there any immediate/obvious positives I could expect, if I change them now?

Any thoughts appreciated muchly!
grin.gif
 
If your O2 sensor is rendered ineffective due to poisoning (very common due to various additives, such as Manganese, MMT, etc.) replacing it with a new one shall restore your car's usual responsiveness, peak fuel economy, proper combustion efficiency (more precise fuel control during close loop, etc.) and reduced emissions.

If your O2 is simply lazy, you shall be able to get back some of your fuel efficiency by have a much more responsive O2 sensor (quicker in entering the close loop).

If your O2 is still functional, replacing it with a new one does little....

Hope that helps.
 
If your Ford is 1996 or newer my suggestion is to leave the O2 sensors alone. The OBD-II system in 1996 and newer vehicles monitor the O2 sensors very closely. If it gets "lazy" or fails outright the ECU will quickly and persistently notify you with a "check engine" light.

1995 and older is a different story. "Lazy" O2 sensors typically won't throw a check engine light until they totally #@$%! out. So there could be a benefit of preventative replacement.
 
Change them if you are going to keep the vehicle. Those high miles are really stretching it.
The rear O2 sensor only monitors that the front one and cat is working. It is not that precise of a system.
It will not be worse , but only better with the new ones in there. How much? Who knows?
 
The computer can only tell if the O2 is out of range or fails to change. But if it's just wrong, but still functional, it can't show a code.
 
The O2 sensor degrades in a linear manner.

The check engine light is a digital (binary) device.

See the problem here?
 
Last edited:
You really need a digital multimeter with a bargraph fuction if you're going to measure the output of the O2 sensor in-car to see how fast it switches. Analog meters don't have a high enough impedance unless they're one of those electronic ones (either a vacuum tube voltmeter or the solid-state equivalent).
 
Quote:


The O2 sensor degrades in a linear manner.

The check engine light is a digital (binary) device.

See the problem here?




Well, having an A/D converter followed by digital processing in between them makes that point sort of moot
wink.gif



O2 sensors are easily one of the most needlessly changed parts on vehicles however if you have a decent meter (a scope would be better) you can test them:

1) A good sensor will cross count a minimum of 8 times in 10 seconds and reach at least 200 mv on the lean side and 800 mv on the rich side while doing it. The midpoint should be right around 450 mv which is stoichiometric.

2) A good sensor will respond within 100 ms when driven from lean to rich or vice versa.

3) You can check them off the engine with a propane torch but it's much easier check them on the engine.

4) If it's OBD-II what Axe Man said.

All that said it wouldn't hurt to change it as it's no doubt near the end of it's useful life.
 
The post-cat o2 sensors only monitor the cats and do nothing for performance or economy. I would only replace those if the car threw a code relating to 'em.

Front ones, what others have said. Just replaced one on my saturn with a bosch universal and the "bog" is gone.

Decide if you want universal (wiring splicing required) or OE fit then shop around... Find a Ford forum to see how well different brands do and if you can get away with a universal one.
 
Thanks guys... I think...
laugh.gif
A lot of this is over my head. I do know a couple things:
CEL comes and goes a LOT, but this became sooooo common in some cruisers, the shop just put black electrical tape over the light.
blush.gif


My MPG seem low, even for a big car...

Anyway, I know from my histroy alone, they have not been done in 80K.

frown.gif
Couldn't hurt I would think...

FYI: The 94's only had ONE cat sensor a side...
 
Quote:


Well, having an A/D converter followed by digital processing in between them makes that point sort of moot
wink.gif





Not really. The O2 sensor has already started to degrade long before that light comes on.

Analogy: It's a similar situation with the SMART feature that hard drives have. Yes, SMART will tell you when the drive is nearing failure. But did you know that with the proper software you can actually watch the parameters that are used to determine when the drive is going to fail start counting down from the first time you power it on? I replaced the hard drive in my laptop because one of the SMART values was very close to the failure threshold. I suppose I could've waited until SMART told me the drive has reached the failure threshold, but what if that happened when I was on a trip or something?

And so it is with oxygen sensors. Of course, the failure of an oxygen sensor isn't going to destroy all of your data and render your car unusable.
wink.gif
 
Last edited:
If his car is OBD-II there is no way the O2 sensor could not be doing it's job and the MIL be off. For a computer type you don't seem to have much faith in the power of code. Nor do you seem aware of how precise OBD II monitoring of lambda is. As Axe Man and others pointed out there is nothing a lambda signal can do that isn't monitored by OBD-II with at least 12 bit precision on the input and a complex algorithm after that. And unlike your hard drive diags the sensitivity of this monitoring is EPA mandated.

The algorithm is left up to the automaker but what it takes to make the MIL come on is standardized and quite tight. Even in one of my older cars (a 1987 import OBD-I system) there isn't a situation in which the sensor isn't monitored for every form of signal fault that would result in emissions exceeding 1.5 times the federal limit, which is what the sensor is there for to begin with. Remember, lambda sensors are not installed to make the engine run "well", they exist solely to serve the catalyst. It's their only purpose in life.

While it's true the sensor will start to degrade from the day it's installed as long as the MIL stays off the sensor is doing it's job because the monitoring code is written specifically to detect otherwise. Since there are a very limited number of things a lambda signal can do in total and even less when operating correctly it's hardly rocket science to precisely monitor it. It's as simple as that.

Now if his engine is pre-OBD-II I'd tend to agree with you because as a rule early domestic OBD was inferior compared to most imports. However even then the MIL should come on well before the sensor degraded to the point where drivabilty would suffer because by then you'd be well past 1.5 times the federal limt.
 
I recall a problem with some Honda OBD-II vehicles where they would not turn on the check engine light when they were supposed to.

I believe this resulted in a recall, and a 150,000 mile
extension of the warranty on the O2 sensors.

Fact is, code is only as good as the ethics and competence of the person who is writing it. (Which of the two was lacking in the Honda case?). EPA regulations, in fact any laws, are only as good as the ability and willingness to enforce it.


As far as domestic OBD goes, both GM and Ford had quite sophisticated systems. GM had a data stream output long before anyone knew what OBD-II is, and Ford's EEC-IV had extensive self-test diagnostics. As one example,, compare the number of possible codes for a typical 1987 Ford EEC-IV system to the number of possible codes for a typical 1987 import system. Then remember there are three types of EEC-IV codes--memory, key on engine running, and key on engine off and that a single code could mean three different things depending upon what type of code it is.

As a final note, I will point out that long-term fuel trim on OBD-II is allowed to go as much as 25% positive or 25% negative before the light comes on and an error code is stored. I think you ought to start being concerned about the problem at 10%, and in fact I became aware of the presence of an otherwise asymptomatic vacuum leak when I looked at the long-term fuel trim on my old car--it was at about 22% correction, as I recall.

Quote:


Remember, lambda sensors are not installed to make the engine run "well", they exist solely to serve the catalyst. It's their only purpose in life.




If it were not for the oxygen sensors, that vacuum leak I described above WOULD have made the engine run like #@$%!. (It has a MAF sensor, not a MAP sensor).

Quote:


Even in one of my older cars (a 1987 import OBD-I system) there isn't a situation in which the sensor isn't monitored for every form of signal fault that would result in emissions exceeding 1.5 times the federal limit




So how much extra fuel is a car using when it is spewing out 1.45 times the federal limit in emissions compared to a car that's only spewing out the federal limit, anyway? (And what's that doing to the cats?)
 
Last edited:
My general rule for replacing heated O2 sensors is every 100K or when O2 sensor related code is generated, whichever comes first.

For unheated sensors--I've never had a car without a heated sensor so I have no rule for those.
 
Brian, we're closer on this than we are apart but you're diverging from the topic at hand. That said I also advocate changing sensors at 100K and will readily admit if it was easy to write properly they wouldn't call it code.

Enforcement is one thing but the EPA has rigorous standards and runs highly detailed testing before approving any vehicle for sale in the US. California goes even further by conducting it's own tests to meet it's even more stringent standards. There is almost zero need for enforcement (beyond testing) in their enhanced areas because the technology employed makes it unnecessary.

We're talking strictly about diagnostics here, not the program that runs the engine. My experience is mid to late 80s imports had better diagnostics than domestics because OBD 1 hadn't been standardized and imports have always lead in technology. Imho they still do. However I'll also admit to far more experience and training with imports and in particular Toyota's TCCS (which I'm knowledgeable on even up through the current model) so you may have me there. I'm a bit puzzled though. You're original point was that his Vic could have a faulty sensor without an MIL but then you claim Ford diags, even the early ones, to be sophisticated.

While your vacuum leak resulted in a lambda issue it was not a lambda *sensor* issue. In older OBD-II MAF systems as long as the leak didn't drive the sensor full lean it'd bring the mixture back to stoich. Had it not you would've gotten a MIL. At least you would've in most imports and based on what you said in domestics also. You didn't, you happened upon the problem by scanning the resulting increase in LTFT instead. The point is there was no MIL because there was no lambda sensor fault and a lambda sensor associated MIL is what we we're talking about. Fwiw shifted loops are better addressed by the latest PCMs but things can't be too tight because nuisance MILs must be avoided.

The amount of richness required to exceed 1.5 times the federal limit can easily be handled by a healthy cat. That's one reason the limit was written there. The upstream sensor would still fault in that case. Unfortunately a problem seldom results in stopping at 1.5 so your point is well made. Mine is there would be an MIL when it crossed 1.5 because that's what the code for the upstream sensor is written to do and verified by EPA and Cali testing.

I still maintain that if the original poster is OBD-II with no major problems and he doesn't have a lambda associated MIL his sensor is keeping the mixture at stoich, same as it did with your vacuum leak, which is all it's designed to do. He should experience little to no related drivability issues and it appears he hasn't any. I also suggested it be changed as it's likely not long for this world. It's just good practice. As I said, we're closer on this than we are apart.
 
Lol. Ulver, Brian and I are both right. If you have a MIL (what you're calling a CEL) you need to see what the code is. Your sensor may indeed be bad. For sure you have some kind of problem
wink.gif
 
Quote:


You're original point was that his Vic could have a faulty sensor without an MIL but then you claim Ford diags, even the early ones, to be sophisticated.




He may well have a borderline sensor--it's 164,000 miles old. That was my original point. The light can tell you "good" or "bad", but it certainly doesn't tell you "how good" or "how bad". (Incidentally, with a good scantool you can indeed access the information from the PCM as to "how good" or "how bad" the sensor is).

As far as the diagnostics in early Ford PCMs, you could actually put them in a special self-test mode which runs tests that are much, much more thorough than the ones it runs during normal driving. I believe their OBD-II PCMs retained this ability, but now it requires more than a paperclip to enable this mode
wink.gif


And they'll run just fine with the MAF unplugged
wink.gif
(Well, I'm not sure about fine, but you can at least drive 'em
that way).

(Also at 164,000 miles, I should point out that the chances of that sensor getting stuck and not coming out increase, too, which is perhaps another reason to change them before they've been in there so long).
 
Let's just say a healthy o2 sensor going down the highway switches rich to lean 30 times in 10 seconds.

Then let's say "they" decide less than 10 times in 10 seconds is "lazy". Furthermore, lets say they run the test every time the car reaches cruise conditions, and make it throw a code only after 3 out of 3 failures in a row.

You can have an o2 that switches 11 times in 10 seconds and pass all tests. It's still not as nice as a "new" one.

I'll take feedback fuel control even if we didn't need any environmental protection. Beats the flush toilet carburetors of old that'd dump a little extra fuel "just in case".
 
Status
Not open for further replies.
Back
Top