Here’s the background.
About a year ago, the Check Engine Light came on in our 2000 Montana. It’s got well over 260,000 on it, so that’s not exactly a big surprise when it happens. Pulling the codes and checking the live data, everything pointed to a simple failure of the B1S2 O2 sensor - the one that sits after the cat, sticking up about halfway back under the vehicle. It’s by far the easier of the two to change on this beast, and it took all of 5 minutes to swap it out. I figured that the sensor, which may have been the original one from the factory, had just finally worn out. (I’d never replaced it before, as far as I could recall.).
About a month later, the CEL was back, and the codes and live data were both showing indications of a bad sensor again. Same symptoms as the first time; the sensor output was at a steady 1.105 V with the sensor connected (which is what the sensor would read if the engine was running so dead rich as to use all of the O2 in the intake stream) and the voltage in the live data dropped to 0 if I unplugged the sensor from the harness. I checked for heater voltage at the plug (it was normal; system voltage across the two pins) and I further tested the harness for shorts - of which I found none. This didn’t preclude the possibility of an intermittent short, of course, but I saw no indication of one when testing, nor did I find any place where the harness looked like it might have been pinched at any time.
Failing to find any other issue, I just chalked it up to “random bad replacement part” and swapped it out again. It was a Bosch sensor that had been installed as a replacement. I’ve never had a defect in one of theirs in the past, and was very surprised that this one seemed to have failed. It took me another couple of months to get around to replacing the failed unit, during which no new codes cropped up, and nothing presented itself as an obvious source of the problem.
Two weeks later, the second replacement was doing the same thing as the first one (and the original, for that matter), and since I was flat broke and out of ideas, and not on board to simply keep assassinating expensive sensors by setting them up like tin cans in a shooting gallery, I put the whole thing on Ignore for a while. Since the downstream sensor is only an emissions inspection thing anyway, and has no bearing on the engine’s operation, ignoring it only produced a financial hazard and not a mechanical one. (And I’ve been lucky so far on the now-expired sticker.)
Anyhow, when I finally went back to the problem last month, knowing that I had a bad sensor in there, I rechecked everything before swapping in a third replacement. And I verified that it was working; the voltages read correctly, and the switching speed and range looked normal. So I drove it. For about three days. And the sensor output first started going intermittently 1.105V steady, and then fixed at 1.105V steady; failed AGAIN.
Now, I have not applied a drop of non-sensor-safe RTV on this thing anywhere, nor did I apply any added anti-seize beyond what Bosch supplied on the threads - and the upstream sensor has soldiered along without a hiccup through the whole debacle. If there was a problem with a coolant leak or contaminated fuel, or excessive oil consumption, or anything of that nature, I’d think that anything in the exhaust stream bad enough to eat three post-converter sensors would have popped the upstream at least once as well. And it hasn’t.
Which brings me to the only thing I can find that might, just possibly, maybe be the cause of the repeated failures. There has been a steadily increasing transmission fluid leak from a source that I finally located today; the o-rings in the snap-together connectors at the radiator weren’t sealing anymore. The airflow under the vehicle has carried the leaking fluid back as far as a couple of feet past the cat under the car, and there’s evidence of it on the heat shield above the cat and the sensor - but the exhaust, and the sensor, which both get really hot in use, show no sign of this contamination. Be that as it may, trans fluid has got lots of Stuff in it, including some dissolved metallic compounds, and I’m starting to wonder if the fumes from the trans fluid burning off of the converter might have been killing the O2 sensors. The upstream sensor is in a location where it would never get a whiff of that stench, which would explain why that one hasn’t failed. But searching the Net has failed to find even a single reference to this being a potential issue, and as often as trans fluid leaks happen, i would have thought that if this was a real possibility, somebody would have run into it and posted a heads-up somewhere. But as far as I can tell, nobody has.
So here’s the poser: Can trans fluid fumes, or trans fluid itself, kill an O2 sensor just by getting to it on the outside? Anybody know for sure?