Thanks for the replies.
It evidently was a bad sensor after all. I tried a third one; that seems to have chased the fault code away.
Why they trigger the CEL
on this car but not my other car, I don't know.
My fuel economy is still terrible in my opinion; around 13 mpg, mostly stop-and-go city type driving, and I'm a conservative driver.
It is smooth at idle as well as any other speed. Compression is very good, no vacuum leaks were found.
I have observed, however, that in the last couple of days, it seems to be developing what feels like an ignition miss. This occurs after about 3 hrs. of driving. I notice it the most at medium speed when the engine is under load. I suspect a possible weak coil or bad wires (they appear to be original).
I suppose that an occasional ignition miss does not enhance fuel economy, but could it cause it to be as bad as I'm getting? I guess I'll find out.