PS - I do believe my heater core is leaking (haven't fully figured if the core is leaking, or the coolant lines to/from the core).

However, it appears to me the fluid in the back is too thick to be water/coolant, and doesn't really smell like coolant - but perhaps the water would have evaporated ...

There's a leak of an oily liquid in the back of the car, near the fuel tank, towards the front of the right-rear wheel.

No idea what it is - other than the fuel tanks, what other liquids are there in the back?

Brake fluid?

Cleared everything out, ran for some time, rechecked:

Now showing LTFT +7.8% (on both). No more codes for O2 sensors.

Checked for vacuum leak - nada (both physical checking for leaks + STFT at idle/1500/3000 rpm - all = no vacuum leak).

Current codes: Catalytic Converters inefficient. This was ...

Finally got the correct sensors, and got around to replacing them.

As you suspected, the old sensors were fine. Ended up getting the same codes shortly after installing the new sensors (and deleting out all old codes).

LTFT is now about 10% for "Bank 2". Something like -3% for "Bank 1".

O2 ...

Got the O2 sensors, set about to replace them - but first disconnected battery. I had read its a good idea to zero-out the LTFT when making engine repairs.

Couldn't replace one of the sensors - wrong model (only difference is a tiny piece of plastic on the plug attachment - right side versus left ...

Yeah, I couldn't make sense of rich reading, with lean fuel trim. Perhaps the ECU is smarter than I give it credit for, and the bad sensor codes are accurate.

I went ahead and ordered two new front sensors. Quite possibly changing out perfectly good sensors - but I don't know another sure-fire way ...

1 thing - during this test, I think the Front Bank 1 sensor was in open loop (scanner shows readings, but car not using data to adjust fueling?).

Front Bank 2 was in closed loop.

OK, another test drive:

LTFT, bank 1: +17.2%
LTFT, bank 2: -5.5%

STFT (both banks) - all over the board, but rarely above 5%

Front O2, bank 1: .90
Front O2, bank 2: 1.10

Rear O2 bank 1: .05 v
Rear O2 bank 2: .70 v

Bank 1 is the passenger side?

Some history:

1) Just replaced spark-plugs - had ...

After exhaustive Googling (without the benefit of having read your response) I found out exactly what you are saying - 02 sensors read 0 - 1 volt, wide-band sensors vary depending upon manufacturer, but European models are typically 1.0, with less than 1.0 being rich, and greater than 1.0 being lean ...

I got scan code ECM-290A (Front Driver-side oxygen sensor, as far as I can find).

Using an Icarsoft i906, I can ready the oxygen sensor datastream. However, I can't make sense of the readouts.

For the rear oxygen sensors, they seem to make sense - steady around 0.6 volts. Initiate a vacuum leak ...