O2 Sensors: Replace @100K or Wait for TBDN

[The_Nuke]

It seems like the popular theory used to be that replacing the (upstream) oxygen sensors wasn't necessary unless they were on the verge of taking The Big Dirt Nap.

Is that still the general consensus (was it ever really?), or has it shifted to the proactive alternative of replacing after normal service life is up?

And what is their normal service life, 100K miles?

Inquiring minds want to know...we'll, one inquiring mind, anyway...who knows what's going on in the rest of those heads out there!

 

[tmorris1]

I've got over 200k on mine and mileage is still the same and now CEL. I say unless you suspect a problem, leave them alone.

 

[CT8]

If the engine runs good and the gas mileage hasn't changed leave them.

 

[slacktide_bitog]

Oxygen sensors do not need to be replaced at a set interval.

 

[Wolf359]

You replace the upstream sensors, leave the downstream ones alone. Replacing them may give you slightly better gas mileage. Like other parts, they do wear out over time. Oxygen sensor makers say to replace them every 60-80k, but you could probably do it anywhere between 80-120k. While they might not throw a code, when they get older, they get lazy.

 

[mclasser]

I've only had one fail on me, setting a CEL at around 180k miles.

You can use a basic scan tool to see if your's are starting to get lazy. But unless you're noticing symptoms -- low MPG, smelly exhaust, rough running, etc.-- I'd just leave things alone.

 

[CELICA_XX]

Just replace them when they go bad.

I had to replace one on a GMC Terrain at maybe 60k (out of warranty)

My Lexus LS is at 300k and still factory original

 

[bdcardinal]

On some cars, a lot of Fords it seems, getting a hard code to set for a bad O2 is about as hard as getting one to set for a misfire sometimes. I think if the HEGOs are cheap enough and easy enough to replace, to go ahead and do it.

I just realized my terms are different, Ford calls upstream sensors EGOs or HEGOs, Exhaust Gas Oxygen sensor or Heated Exhaust Gas Oxygen sesnor. Downstream sensors are called cat monitors. I wouldn't bother replacing a cat monitor unless you replace a catcon which if you do that you might as well do the HEGO too.

 

[14Accent]

I had a funny one come in the other day: 2002 Saturn LW300 with the 3.0 V6. Little old lady car, around 90k miles.

The only codes were P0174 - system lean bank 2 and P0300 random misfire. I found it extremely odd that only one bank would be running lean, but a quick check of the data showed that the fuel trims on bank 1 were +/- 2% while bank 2 was maxed at almost +30%.

This car uses wide-band air/fuel ratio sensors in the upstream spots, and they're tougher to watch than regular sensors, but what I noticed was that the B2 upstream was stuck indicating basically as lean as it could read, but the downstream was maxed at 900mv (full rich). One of them was lying... Guess which one? I pulled the upstream sensor and it was completely fouled. I replaced it and reset the fuel trims, everything was a-ok.

Moral of the story: sometimes a bad sensor can hide in plain sight.

 

[buck91]

I have looked at the oscilloscope read out on older vehicles to make sure they are switching well but I would expect newer cars computers to be far more effective at that than we are.

 

[The_Nuke]

Originally Posted by 14Accent
I had a funny one come in the other day: 2002 Saturn LW300 with the 3.0 V6. Little old lady car, around 90k miles.

The only codes were P0174 - system lean bank 2 and P0300 random misfire. I found it extremely odd that only one bank would be running lean, but a quick check of the data showed that the fuel trims on bank 1 were +/- 2% while bank 2 was maxed at almost +30%.

This car uses wide-band air/fuel ratio sensors in the upstream spots, and they're tougher to watch than regular sensors, but what I noticed was that the B2 upstream was stuck indicating basically as lean as it could read, but the downstream was maxed at 900mv (full rich). One of them was lying... Guess which one? I pulled the upstream sensor and it was completely fouled. I replaced it and reset the fuel trims, everything was a-ok.

Moral of the story: sometimes a bad sensor can hide in plain sight.


Is that normal that one of the upstream sensors can get completely fouled to the point of causing the other problems you detailed, but the other upstream o2 sensor is not similarly fouled? (Are there 2 upstreams on it?)

And what would be the likely source of that sensor's fouling, in your opinion?

 

[14Accent]

Originally Posted by The_Nuke
Originally Posted by 14Accent
I had a funny one come in the other day: 2002 Saturn LW300 with the 3.0 V6. Little old lady car, around 90k miles.

The only codes were P0174 - system lean bank 2 and P0300 random misfire. I found it extremely odd that only one bank would be running lean, but a quick check of the data showed that the fuel trims on bank 1 were +/- 2% while bank 2 was maxed at almost +30%.

This car uses wide-band air/fuel ratio sensors in the upstream spots, and they're tougher to watch than regular sensors, but what I noticed was that the B2 upstream was stuck indicating basically as lean as it could read, but the downstream was maxed at 900mv (full rich). One of them was lying... Guess which one? I pulled the upstream sensor and it was completely fouled. I replaced it and reset the fuel trims, everything was a-ok.

Moral of the story: sometimes a bad sensor can hide in plain sight.


Is that normal that one of the upstream sensors can get completely fouled to the point of causing the other problems you detailed, but the other upstream o2 sensor is not similarly fouled? (Are there 2 upstreams on it?)

And what would be the likely source of that sensor's fouling, in your opinion?


Yes, there's 2 upstream and 2 downstream. The sensor was fouled as a direct result of it's failing: it kept telling the computer the mixture was lean, so to compensate the computer kept adding fuel until it was out of correction room. The rich mixture is what fouled the sensor, but the sensor failing first is what caused the rich condition.

 

[The_Nuke]

That doesn't bode well for the health of that cat does it? If the system was able to get enough fuel thru to foul that sensor, the cat behind it would have been getting the shower of fuel, and that's never good.

 

[Wolf359]

Originally Posted by The_Nuke
That doesn't bode well for the health of that cat does it? If the system was able to get enough fuel thru to foul that sensor, the cat behind it would have been getting the shower of fuel, and that's never good.


Which is why it's a good idea to replace them before they go bad based on mileage.

 

[14Accent]

Originally Posted by The_Nuke
That doesn't bode well for the health of that cat does it? If the system was able to get enough fuel thru to foul that sensor, the cat behind it would have been getting the shower of fuel, and that's never good.


Very fair point. I'm quite sure then cat didn't like being saturated in fuel, and I wouldn't be surprised to see the car come back with cat codes in the future. But, I confirmed O2 operation after I got it running right and the downstream sensors we're both hovering between 400-600mV, which is just about spot on.

 

[wtd]

I usually replace them by mileage which for the four wire sensors is at 100,000 miles. Every time I have replaced them, my gas mileage has increased. The single wire sensor in my 92 cavalier I replace at 30,000 miles.

 

[BAJA_05]

Just replaced one on the Subaru in signature two weeks ago -- right before 96 thousand miles. Was throwing intermediate code/check engine light for the last 1500 miles probably, exhaust really rank & was not running smoothly or normal should I say.


Dale