Scheduled Replacement of O2 sensors

[PandaBear]

I change mine usually when my smog check result is getting worse and almost failing. Usually that's at 150k first and then around 240k. Again, I know where to buy (rockauto for example), and what to buy (those cheap but still Denso / NTK core aftermarket brand) for only around $20-30 ea. If I had to pay $300 to replace it at a shop, I'd probably wait till I fail smog or have a CEL on.

 

[Wolf359]

Originally Posted by maxdustington

They're too expensive to make up for potential fuel economy gains. There's no way $150+ is going to be recouped in fuel if you replace them at 100k. You might not even own it by 200k. We're talking what? 1-3 MPG gain that will taper off as the new O2 sensor ages.

I think you have to do the math. Say you get 1 mpg more over 25k and gas is $5 a gallon and you get 21mpg instead of 20. So a 5% improvement would save you about $300 in gas. If you got 2 mpg higher or it was over 50k then your savings would be higher. Of course if you were getting 30mpg and only got 1 or 2 mpg more, then your savings is lower.

 

[HangFire]

Fuel economy goes up in Summer for two reasons. No more winter formulation gasoline, and less dense hot air requires less gasoline to make stoichiometric. Still, 18 to 20 is a good jump up. I was considering a proactive sensor change on the Honda at 200+K miles, but the transmission died first. It used to get 20 and was getting 17mpg on average.

 

[The_Nuke]

I'll buy it goes up when a MAF is calling the plays, but not for an engine that still uses the abacus of air/fuel mixture captains: MAP - brought to you by Speed Density. Now with more Heatsoak!! The next time my IAT is even within 15F of the actual air temp its mixing fuel with will be the first time. Throw in an August day in Texas traffic, and I've got a air/fuel mixture perfectly mixed...for driving around on the Sun.

 

[Deontologist]

Replacing O2 sensors at 100-150k might not be that bad of an idea. When I worked at O'Reilly, I remember printing out a graph for a lady who had come in. Her LS460 was throwing some oxygen sensor codes. The graph showed that around 100k miles, oxygen sensor failure rates started climbing uphill, and by 140k, the number of people who had reported O2 sensor replacement hit its peak. The graph fell after 140k. And this was for a LS460, which probably only uses the highest quality Denso O2 sensors. At 100k, it's probably just a matter of time.

 

[gathermewool]

I would prefer to monitor fuel trims than arbitrarily replace a sensor. If my MAF and O2 sensor diverge over the years and I can't find a vacuum leak, then I'd be leaning toward cleaning the MAF and then replacing a sensor or two with age. If I'm at 400k miles and my LTFT is within +/- 4% I'm not touching either sensor...

 

[The_Nuke]

I had planned on quoting a specific reply with this reply, but I don't see it now, so I will just drop this in at the end here for anyone that wants to use it: "Once you know how to check an oxygen sensor, you can verify whether the unit is actually working or in need of replacement. These tests will save you money and time—and help you fix your vehicle sooner. Also, you might want to check the service interval for your O2 sensor in your repair manual. As driving miles accumulate, exhaust byproducts cover the tip of the unit that protects the sensing element. Then the sensor efficiency drops, causing engine performance issues. Eventually, the sensor stops working. So it's a good idea to replace it at the recommended schedule to restore fuel efficiency and reduce emissions." https://youtu.be/NIzcztO-CCs Copied from this site: https://axleaddict.com/auto-repair/How-to-Check-an-Oxygen-Sensor

 

[RayCJ]

FWIW, here's my theory. US Federal law requires all emission components to have a minimum lifetime. I think O2 sensors for passenger vehicles are 100,000 miles (40 CFR § 1037.120). MTBF is Mean Time Between Failure. If a sample set of sensors has an MTBF of 100k miles, you can be certain that 50% will fail before 100k miles and 50% will last 100k miles or longer. The auto manufacturers do not want the hassle of such a high probability of doing warranty work so, the sensors are designed with a much higher MTBF. Based on a standard distribution of defect rate, I'll guess and bet dollars to donuts they shoot for an MTBF of 300k or 400k miles. Doing so pretty much guarantees most will live up to the 100k mark and spare them from doing required warranty work. On the other side of that coin, it won't be uncommon to see them lasting 300k to 400k miles. BTW, I've only had one car in my lifetime require an O2 sensor and it intermittently (momentarily, once a month) gave CEL indications at 105k miles. At 120k miles, the CEL light was more frequent at which time, I replaced the downstream sensor. At no time were there any signs of rough running etc... I never gave much thought proactively checking O2 sensors but, its on my radar now. Ray

 

[Egg_Head]

Originally Posted by The_Nuke

Originally Posted by Egg_Head

Maybe true for OBD-I cars pre-1995, I would "think" OBD-II systems can better detect when an O2 sensor is getting flaky. I'd wait for the code.

How would the PCM alert the driver to this flaky condition? Set a DTC with a message like "slow responses detected; prepare for sensor failure at some point in the future"?? No such codes exist that I know of.

Maybe with one of these? https://repairpal.com/obd-ii-code-chart/p2axx

 

[The_Nuke]

Originally Posted by Egg_Head

Maybe with one of these? https://repairpal.com/obd-ii-code-chart/p2axx

That information is as useless as it is informative...and it's pretty informative I will admit. I didn't know those existed, and neither will anyone else because they don't meet the criteria for sensor failure. As such, they won't illuminate the CEL, and thus can exist in the PCM for days, weeks, or even years without the driver knowing about them. My point is, depending upon the car to alert to inefficient, yet still functional O2 sensors is setting yourself up for failure UNLESS you actively check for the existence of such codes on a regular basis. How many of us do that? How many of the general public who own and operate cars do that?

 

[pitzel]

Originally Posted by RayCJ

FWIW, here's my theory. US Federal law requires all emission components to have a minimum lifetime. I think O2 sensors for passenger vehicles are 100,000 miles (40 CFR § 1037.120). MTBF is Mean Time Between Failure. If a sample set of sensors has an MTBF of 100k miles, you can be certain that 50% will fail before 100k miles and 50% will last 100k miles or longer. The auto manufacturers do not want the hassle of such a high probability of doing warranty work so, the sensors are designed with a much higher MTBF. Based on a standard distribution of defect rate, I'll guess and bet dollars to donuts they shoot for an MTBF of 300k or 400k miles. Doing so pretty much guarantees most will live up to the 100k mark and spare them from doing required warranty work. On the other side of that coin, it won't be uncommon to see them lasting 300k to 400k miles.

I don't think O2 sensors are items that auto makers can arbitrarily engineer a long life into, by using extra materials, etc. My belief is that the automakers generally pick the best technology they can find for O2 sensors (or that sort of equipment), and then they have to massage the ECU software in such a way to facilitate getting as many vehicles out of warranty as possible without raising a CEL that would trigger an in-emissions warranty repair. It would be easy to just set very tight thresholds for catching O2 sensor degradation/failure early, but then they'd be dealing with a significant number of vehicles at 80-90k I reckon, as certainly there are people who drive mostly city, as opposed to highway-only vehicles where an equivalent amount of fuel and cumulative engine revs would take a car 150k. I personally theorize that the regime in which an O2 sensor reads overly lean (causing an engine to richen), but beneath the threshold of triggering a CEL-driven maintenance action places a significant additional burden on the catalytic converter, potentially causing its premature failure. And those aren't cheap items either!

Quote

If I'm at 400k miles and my LTFT is within +/- 4% I'm not touching either sensor...

Interesting, I don't have a full and proper scan tool, but I do have one of those Bluetooth OBD2 things, and the Torque app on my smartphone. If the LTFT drops upon installing the new sensors, then I would hence have proof of efficacy (does LTFT directly correspond to specific fuel burn???). Is LTFT definitively diagnostic for oxygen sensor degradation? Know of anyone who has meticulously monitored and recorded their LTFT numbers over time and posted the results online?

 

[CBR.worm]

In the past, I would replace O2 sensors around 90K miles. I am currently at 105K on my O2 sensors in one of my vehicles and am debating changing them now. Using Denso or NTK replacements for the upstream sensors would be $300 - so less than a month of fuel for that car, but still enough to prefer not to change them until I need to. I've been following the same basic practice since the 80s and I am coming to realize that I may over-maintain things that last longer now than they used to. Like every part on a car.

 

[Wolf359]

[quote=pitzel] I don't think O2 sensors are items that auto makers can arbitrarily engineer a long life into, by using extra materials, etc. My belief is that the automakers generally pick the best technology they can find for O2 sensors (or that sort of equipment), and then they have to massage the ECU software in such a way to facilitate getting as many vehicles out of warranty as possible without raising a CEL that would trigger an in-emissions warranty repair. It would be easy to just set very tight thresholds for catching O2 sensor degradation/failure early, but then they'd be dealing with a significant number of vehicles at 80-90k I reckon, as certainly there are people who drive mostly city, as opposed to highway-only vehicles where an equivalent amount of fuel and cumulative engine revs would take a car 150k. I personally theorize that the regime in which an O2 sensor reads overly lean (causing an engine to richen), but beneath the threshold of triggering a CEL-driven maintenance action places a significant additional burden on the catalytic converter, potentially causing its premature failure. And those aren't cheap items either! [quote] Yeah, it's not like the automakers design O2 sensors. It's various vendors that actually design them which is why you find Bosch and other oxygen sensor makers in so many other cars. Car makers really just assemble a bunch of parts. The part makers try to come up with parts that they will buy but you're still limited by physics. Most of the time when you do hear of them failing they're in the 100-120k range, never really heard of anyone getting 300-400k out of one. But I suppose that would be possible. If you ran it in steady state long enough, there might be a chance. But probably normal driving with all those start/stop cycles, heating and cooling probably lead to a much lower average life.