Oxygen Sensor?
 

My Hyundai Accent has 160K miles. I've had it since 86K on the odo.

A couple months ago, the Check Engine light came on. Pulled the code and IIRC it was an 02 sensor code. I reset it and the light didn't come back on for a month, so I figured it was a stray data # that came through the ECU or something. Light popped on after that month or so. (I drive 2400 miles+/month) So I again cleared it. That was a week ago.

I have a Scangauge now, and after several hundred miles, it still shows the ECU as "Not Ready." How long should this take!?

Either way, I'm assuming from what I know now from what I've read here that the O2 sensor is likely on it's way out? (I've never replaced either of them on this car)

Reason I really suspect this is I've been driving MUCH better for FE, yet see no improvement. I get ~36 MPG over time whether I do 72 or 60. That does NOT seem right at all. In the past when I would even slow down to 67 instead of 70-72 and take it easy, I could get 39-40 MPG easy.

Does it make sense that a near-bad 02 sensor could cause this, but not throw on the Check Engine Light right away?

Sometimes they don't throw codes for the O2 sensor per-se, but will come up with stuff like...

"Injector pulse width too low"

Meaning, "Hey this here sensor is telling me it's lean, but somehow I can't seem to hose enough gas in to richen it"

But it sounds like you're not actually in closed loop mode, this could be for several reasons, like the coolant temp sensor being bad, or the MAP/MAF being bad.

SG says I am in closed loop. As of now, the light is not on, but it's been a week since I reset it and the SG still says not ready.

I would suggest you go ahead and replace the Oxygen sensor. As they get on, their responsiveness to changes slow down and they don't necessarily get bad enough to throw a hard fault, most of the time, but they don't work right, either.