2004 V70 - DTC 4201 - O2 Sensor Graph Reading Assist Requested

[packetfire]

So, I keep getting DTC ECM-4201: "Three-way catalytic converter (TWC) efficiency - Power deterioration". But the car runs fine, no other codes, nothing wrong with fuel/oxygen mix, so it seems to be a sensor.

So, cleared the error, drove 1.5 hrs to Philly in the rain, and halfway back, the Check-Engine light comes back on, and the rear O2 sensor does this, something I've not seen before:

o2_sensors_at_CEL.png (71.86 KiB) Viewed 563 times

The sine wave on the rear O2 sensor voltage happened just when the CEL came on. It apparently stopped when I decelerated to pull over to the side and stop the O2 graphing, the pegged readings was when I stopped, and the resumed readings where "at idle" with the CEL.

Driving along with cruise control at 65mph, I had what seemed a very calm set of graphs:

o2_sensors_on_cruise_control_65mph.png (48.34 KiB) Viewed 563 times

And, when using the brakes and throttle myself, cruise control having clicked off when I did so, I had also "normal-looking" graphs:

o2_sensors_manual_throttle.png (44.46 KiB) Viewed 561 times

Or am I missing something here? My guess is that the rear O2 sensor goes wacky after heating up and being used for a while, and needs to be replaced, but I'd welcome other views, as this is my first O2 sensor issue on the V70, and the car I knew best was my 1982 240 "brick" wagon, which had a far simpler set of emission controls.

[jonesg]

i don't know but i'd swap the sensor.