There has been a lot of discussion recently on O2 sensors and fuel trim, I was wondering if it is a good idea to just replace them say at 100000 miles or wait for codes? I have read that the response gets lazy as they age but maybe not enough to throw a code. These sensors are positioned in the exhaust stream a very inhospitable place to be. Would I even notice a difference such as better gas mileage, smoother running by replacing them? Would replacing the upstream sensors first, make sense?

 

It's likely different depending on car MY (before or after redesign 2002.5MY/2003USA). Before then the upstreams are old tech and yes can get lazy. After that they're wideband (aka linear aka AFR) and not so likely to fail without codes.

If you replace them keep the old ones! Some of the replacements are poor quality and fail soon.

A way to get more info is measure the actual exhaust gas (done at MoT here every year for cars >3 yrs old, but USA etc have similar tools for the measurement).

 

Thanks for the reply, I have a 2005 STR and if I did replace them I would buy Denso sensors which I believe are original equipment.

 

One of my STR's upstream ones failed at 122K miles, gave a code (P1646/7 sorry I forget which) but others still OK.

 

Interesting question. Back in the dark ages of emissions control, when O2 sensors were becoming fairly widespread (mid 80s or so), they definitely had a finite life. Many manufacturers recommended replacement every 30k miles or so. The sensors themselves just weren't that robust, and they lived in a very hostile environment, rapidly fluctuating back and forth between rich and lean. Even with the best systems of the day, air/fuel ratio control just wasn't very precise. Monitoring of the sensors themselves was crude at best.

Fast forward to recent times, and the durability and monitoring for the sensors has increased dramatically. In theory, when an O2 sensor starts to fail, the computer should be able to realize this and flag a fault code. However, this is a bit of a double-edged sword, so you do have to be careful. More in a minute. The computer makes the bulk of fuel delivery decisions based on feedback from the O2 sensors. Other sensors are in play, too, but the lion's share is based on the O2 sensors. If an O2 sensor's output starts to become inaccurate, in theory the computer can catch this. The computer has stored data tables of what it should see under certain conditions, i.e. at this RPM and throttle position, then X amount of fuel should be required and the O2 sensor should report Y value. For the most part, if an individual sensor is outside the expected range, the computer is usually pretty good at identifying the discrepant input and throwing a fault code.

Note I said usually. As smart as computers can be, they only respond as programmed and can be quite stupid at times. For example, an O2 sensor falsely reading lean could mimic the conditions for a P0171/P0174 lean code. The computer responds exactly as programmed, and increases fuel to compensate for a perceived (but non-existant) lean condition. Once the long term fuel trim increases enough, you may get the code. Meanwhile, typical troubleshooting for these codes has you start by checking for vacuum leaks, which can also cause a lean condition. It's easy to head off down a rabbit hole, so you do have to be careful when interpreting fault codes. I'd suggest keeping an eye on the long term fuel trims to get an idea of O2 sensor accuracy.

So should you replace O2 sensors on a regular basis? In my over-inflated opinion, I don't think it really necessary on later model cars. An '86 Yugo? Certainly. But not so much on our cars. If the engine is running well with no fault codes, and long term fuel trims are good, the O2 sensors are probably still good. But if you do have a problem, keep in the back of your mind that some seemingly unrelated faults could be caused by a bad O2 sensor, at least partially. If the fault is restricted to one side, I'd be tempted to swap side to side and see if the fault follows. For the early models, a good name brand narrow-band O2 sensor is only around $40, so it's not a huge gamble to try one. On later models, a wide-band design was introduced, and these run about $200 each for the upstream sensors. (The downstream sensors are still the less expensive type) As an honorary Scotsman, I'd be hard-pressed to spring for that kind of money unless really needed.

For the downstream sensors, a special note: These sensors primarily monitor catalyst efficiency, making sure the dirty exhaust is getting cleaned up. Relatively dirty exhaust enters the cat and is measured by the upstream sensor. The upstream sensor is constantly seeing fluctuations rich and lean as the computer rapidly adjusts fueling. These fluctuations are normal and indicate this feedback loop is operating as designed. Next, relatively clean air exits the cat, and is monitored by the downstream sensor. The downstream sensor should have a fairly stable output, indicating a steady stream of relatively clean exhaust. If the downstream sensor fluctuates at a rate similar to the upstream sensor, a P0420/P0430 code (catalyst efficiency) is flagged. As an O2 sensor ages, its response rate slows down. A slow, lazy sensor is good to have downstream, as it is less likely to flag a code. So unless you have a code directly faulting a downstream sensor, I do NOT recommend replacing them.

Have I bored you to tears yet? Sorry you asked? For the history of O2 sensors, I decided to be a bit more concise and did not go into details of how the critical rare earth elements came to be as the earth was formed. I just went straight to the part where O2 sensors were originally installed on vehicles. You're welcome. And on a practical note, most times when I have a fault code for an O2 sensor (multiple vehicles), it is for the sensor's heater circuit. The actual sensor circuit was still working just fine, but the internal heater burned out.

 

Thanks for all the info. Karl, I am never bored by any of the responses I get on this forum, I enjoy learning from informed members.
P.S. I do enjoy your humor.

 

Thank you, but it's generally considered a breach of forum etiquette to compliment me like that. I'm sure other long-suffering members will be quick to warn you that it only encourages me...