hm. good to know. the top primary heater circuit (gotta love scantools!)
O2 on my 98 hatch took a dump last year and i replaced it with one from
the local stealership. $220, but they were the only people to have one
in stock. autozone and kragen- both out. sh.t- forgot to try napa :( i
didnt wanna wait for mail order and run the risk of screwing the
catalyst up. besides, it was stalling at idle.

just had the biannual smog check, and almost all the numbers are *lower*
than the last time! a couple are a tick or 2 higher, but all are below
average, and wayyyy below the limit. 2 years ago, i had some numbers
that were pretty near the limit.

You can Blame the o2 sensor, but what else is wrong? Regardless of what
brand they are they work on the same principal, and the numbers won't effect
the economy unless dead or off. here is something I wrote concerning this
subject.

OK, Here it goes 02 class 101 (and catalyst efficiency)

The O2 sensor generates its own voltage, and without my meter I couldn't
tell you, BUT trust your scanner if it gives you a voltage reading. The
first o2 sensor switches when the engines running, you watch it go low to
high (sometimes .020 volts to a high of .900 volts (Milivolts??)
It should click along at a steady pace, picking random numbers into the
700-900 range then low again in the 100's
It is looking for Oxygen. This sensors job is to tune the car if it starts
to run to rich; i.e., if too much fuel starts feeding into the cylinders,
for whatever reason, the o2 sensor will start showing a higher range of
numbers, and the computer will start decreasing the injector pulse to
decrease the amount of fuel.
Anyway, a 02 sensor can get sluggish or it can freeze at a random number
then it lies to the computer that will still try to correct it. but usually
this sets a 02 code...

Now that basically describes the 02 sensors operation. The 2nd o2 sensor's
job is to check to see if the cat converter is doing it's job (Generally,
most 02 sensors are the exact same part excepting for the plug and wire
length). It also switches the same as the first one but at a slower or
reduced rate. If your scan tool shows the aft 02 sensor stuck, I'd try it
first.
If your second (aft) 02 sensor is switching at the same pace as the first o2
sensor (the numbers won't match) then chances are the cat is bad.

Now if you are having a fuel consumption problem you need to look at the
computer and look at the short term and long term fuel trim. This tells you
if the car has been trying to adjust the fuel delivery based on all the
other data it gets. If you hook up a scan tool and watch the O2 sensor work,
and it's going from low to high and back again, then it's doing it's job and
no other brand o2 sensor will do it any better.
You can't take one part and blame everything on it without knowing how it
works and works with the car. I've been to many classes on this stuff and
still am trying to sort it all out, got another class on Monday and Tuesday
night. We will be looking at graphing lab scopes analyzing the injector
pulse width and the ignition spark in reference to trouble shooting
misfires. Good thing there isn't a test....