2000 i30 o2 sensor reading (very technical advice requested)

[si2ki30]

I recently had a P0430 code come up on my 2000 i30 with 90K miles. (low catalyst efficiency in bank 2)

Now I don't want to just go and waste money replacing the bank2 cat and sensors (but will do it since they are all up there in age and miles, so might as well do it if they are the real culprits).

So I want to see if anyone familiar with reading o2 data sensor scans can explain what is going on in the scan picture I have below.

After keeping the RPM elevated at about 2,000, I dropped the engine back down to idle.

From the way I understand it:

Bank 1 shows sensor 1 going back into it's normal fluctuation as the ECU tries to keep the bank 1 side of the engine as close to 14.7 A/F ratio as possible.
Bank 1 sensor show the oxygen level at a fairly steady (compared to sensor 1) level after the bank 1 catalyst has done it's job.
(Seems fairly normal to me)

Bank 2 shows sensor 1 going back into it's normal fluctuation as the ECU tries to keep the bank 2 side of the engine as close to 14.7 A/F ratio as possible.
However, sensor 2 goes into "following" sensor 1 fluctuation, but then goes back to showing an almost even rate, but still dipping a lot more than Bank 1/Sensor2
a) why does sensor 2 start fluctuating suddenly?
b) then it stops flucuating as bad, but why is it still not as "steady"

Also, Bank 2/Sensor 1's high peaks seem a lot more "rounded" compared to Bank 1/Sensor 1's peaks.
c) What does this indicate, if anything at all?

Up until the car is warmed up past 90%, it may very infrequently stumble very slightly. I haven't caught this RPM stumble on a datascan (there's no RPM dip in this scan)
Until the car is warmed up, the after-cat sensors don't show anything in a datascan anyway.
d)wondering if this stumbling has anything to do with my o2/cat questions.

Very large image here:

[the converted]

The sensor looks like it's functioning fine and the cat is in fact does look bad. Think of the cat as an expansion chamber or a capacitor in it's functions as well as cleaning up emissions by burning the unburnt fuel in the exhaust. In addition to leaning out what the sensor sees, it will also smooth the fluctuations as well.

If you want to replace the cat, that should fix the code, or if you are being cheap you could always do the old spark plug anti-fouler trick.

[loystock]

The Rear O2 sensor should normally lag the Front O2 sensor due to O2 storage capability of a GOOD 'cat.' If the Rear O2 sensor follows the Front O2 sensor signal, that means the 'cat' has failed.