It's definitely not going to blow. It's not running a 20:1 AFR. It definitely would have blown up already. Also, thanks about the air pump -- I googled that and found out that it isn't present on a 04, as you said (scotty). Last time it was rebuilt by a dealership that can't tell a fan from a turbocharger it had audible knock. I told my buddy it was going to blow. This thing is running good. Before the sensor gives out I can see clear 11:1 AFR readings under full throttle. I know the stock sensor isn't 100% accurate, but I was at autozone today and took a picture of the screen -- bosch calls it a "wideband" sensor -- I see this is quite the hotly debated subject on whether or not it actually *is* wideband.
The problem seems to be getting worse. I'm leaning more towards a defective sensor. It's starting to duplicate this behavior off-highway now that I've got four or five tanks of gas through it. If it were lean, I wouldn't be getting 122 miles to a tank. The best I've done yet is a bit over 200, I think 228mi for 16 gallons.
I hate the fact that it might be the ECU. I find it really hard to believe that the damn ecu could go bad... I have 386 PC's that still work fine, and coming from the honda world about ten years back, it's a shock to see the diagnostics procedure be something along the lines of "check sensor, check wiring, replace ECU". I'd figure if it were "bad" the car wouldn't run. How does an ECU go "kind of" bad? Where's it even located on a Subaru? Is this common? How much for another and how does one go about getting one? eBay?
It's not the sensor wiring as I've gone over it end to end 100 times this week and weekend. I ended up trying unplugging the rear 02 to no avail (no change, same crap, then a CEL) -- I fixed the midpipe exhaust leak but like you guys said I figured that wouldn't matter.
There's no other changes from the way it was built, but who knows -- these no-name equal length headers (though not leaking) could be causing an issue. I was looking at them and I noticed I really disliked the way the bung was placed. It almost holds the sensor too far out of the pipe in my opinion, which may be part of the problem.
I'm figuring there *has* to be a warranty on this part that I bought from the dealer so I'm going to get it exchanged this week and rule it out. It's very possible I just could have gotten a bad one. I know one 3rd or 4th gear pull would have blown the thing if it was running anything near 20:1 AFR and it isn't.
I'll get the UEGO in next weekend and verify that once and for all.
Thanks everyone for your input so far,