Replaced the sensors 70k miles ago but feel like getting a dead-sensor code is a little premature at that interval. I don’t know where I first shaped this expectation, but I usually expect O2 sensors to last about 100k miles, right? Or is this normal for Discos?

 

I think they should go a very long time. I don’t think they should typically fail outright. In my view, they would typically fade in the direction of running a bit richer over time.

 

O2 sensors can get dirty (contaminated) or overheated, or the wires get oil soaked so no air can pass. So, answer depends on how well the engine is running, and how well it was put together. 70k seems pretty good to me, but 100k seems more reasonable.

Signal inactivity could also mean the bank is stuck rich or lean, so good to check fuel trim values too, but first option is to change or swap sensor. Also check signal wiring.

 

Honestly, they're inexpensive enough, the engine is at 210K miles, maybe I'll just swap out all four and give it a fresh set. AB has a set of four Walkers for about $250.

 

I’m not sure that there’s ever a reason for replacing the downstream sensors if you aren’t getting a code for the Cats.

 

I'm with Harvlr on the downstreams. They only report the effectiveness of the cats. The upstreams affect engine management and are far more important. And, IMO 70k is a decent life so replacing them isn't a crazy idea. They're definitely on their way out at that point. If you're lucky enough to be able to do the fronts in situ then I'm not sure I'd go the extra mile on the rears. If the Y pipe is coming down then if nothing else take the rears out and put them back in just to exercise the threads. My last go round with replacing them (I ended up doing all four...) was nearly a much bigger job as the threads were pretty well galled and there was no way the fronts were coming off with a spanner. I had to drop the pipe and remove them with an impact wrench which made a mess of the threads. I got lucky and was able to recut the threads, otherwise it would have meant welding new bungs on. Not a horrible idea since it gives you the opportunity to mount them at a more convenient angle for future removal, but not the job I was looking to get into that evening.

 

Don't forget to put some anti-seize on the threads. I got lucky taking one of the two out. The other was really on there and left me with a bloody knuckle after the wrench slipped. Make sure to check that the sensor wires are tucked away and not just hanging freely. Mine had come out of its clip and had oil running down it. Still trying to figure out where that oil came from....... oh well.

I have heard of people cleaning off the sensors with gas. Not sure how well it works or if it has side effects. But with how cheap they are you might as well just replace them. I got mine at O'Reilly's for either $34 or $64, cant' remember.

 

buy the best quality sensors - recommend Bosch / NGK - the way parts are made today you want to avoid replacing often

 

I just noticed that RockAuto has Bosch 15738 Universals (fronts) on Closeout for $30/ea. You have to crimp your old connectors onto them so it's kinda hokey but I've used these plenty of times and as long as you don't manage to screw up the connecting part they work perfectly fine. I just bought a pair to keep on the shelf.

https://www.rockauto.com/en/parts/bo...2)+sensor,5132

 

Appreciate the heads up! I ordered two upstream NTK's from RockAuto a couple days ago. I'll let ya all know if that resolves the codes when I get them in.