I am in the process of diagnosing a P0420 (that repeatedly came back after erasures) on my 2000 Corolla. It has original O2 sensors and catalytic converters. The MAF sensor was a few year old and cleaned with electrical cleaner a few months ago. It also loses engine oil after driving (around 1 Qt per few hundred miles), but there is visible sign of oil on the out side of the engine, probably due to old gaskets.
I am using a simple scan tool that shows numbers but cannot graph.
The measurements described are taken a few minutes after the engine starts with engine spinning at around 2500 RPM.
- O2 sensor 1 (Is this upstream?) is oscillating between around 0.1 V to 0.8 V.
- O2 sensor 2 (Is this downstream?) was quite stable around 0.67V to 0.8V (with occasional excursion to low voltages; the excursions seem a little correlated with sensor 1, but not nearly as frequent)
Can anyone advice me on how to interpret these findings, especially the sensor 2 reading, and give me an opinion on what could be bad and what other tests I should perform?
I have posted images of live data from my scan tool in: https://www.toyotanation.com/threads/2000-corolla-voltage-of-downstream-o2-sensor.1663964/#post-14114201 (this forum's attachment size limit is too small for those images).