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Originally Posted by theadmiral976
Did you replace with an OEM sensor? Similar to you, I've been noticing my commanded AFR trending down from ~14.7 during long drives at constant RPM. Specifically, the downward spiral starts at the 1.5 hour mark and drops ~0.3 units every 20 min thereafter. For me, my STFTs and commanded AFR haven't hit 0.0 and ~12.2 respectively, indicating open loop; however, I imagine that I'm headed for that soon enough.
I wonder why these rear O2 sensors are dying. Perhaps the altered heat transfer of the uncatted header (I have an OFH)? If so, I wonder if there are better aftermarket options for the O2 sensor?
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I replaced with an aftermarket Bosch, seems to work fine. It definitely sounds like you're on your way to open loop. Peculiar but it seems there are a few of us that are seeing the same behavior.
I was wondering why it would have failed too. I'm only at 32k miles and never hit or banged the sensor - I did swap it in a few times since I went through 3 leaky borla header warranty replacements before I finally got the OFH, although I never even had to apply that much torque to swap the O2 sensor. Since there is no cat it's getting hit with hotter/dirtier air, so maybe that does have something to do with it. Although if it was that I'd think we'd start seeing more failures from everyone who's running a catless header (or maybe they're just not monitoring AFR constantly). I guess time will tell how much longer the new O2 sensor lasts.