2000 Corolla --- voltage of downstream O2 sensor - Toyota Corolla, Chevy Prizm (1998-2008)

7 posts

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).

Thanks,

320 posts

I think sensor 2 is downstream, that is, after the catalytic converter. In theory it shouldn't vary too much with sensor 1. With that much oil leakage have you pulled the plugs to see if one of them is fouled with oil? That could cause problems. Any blue smoke? (Really, that should be fixed, that's way too much oil use). Pretty easy to replace the oxygen sensors. If none of the plugs show sign of taking an oil bath, and you're pretty sure there's no real internal leakage, I'd probably change sensor 1. If that fails, sensor 2... last time I looked they were $40 and 30 minutes to replace.

7 posts

I have corrected spark plug oil bath problem (along with plug replacements with NGK laser iridium) a few months before P0420 came on. No blue smoke. (I have replaced the valve cover gasket already, but that did not stop all the leak.)

One detail I neglected to mention. Right before P0420 came on, the front section the exhaust pipe, the part that is upstream of the catalytic converter, was replaced at a shop. What prompted me to do so was exhaust leak that I had been smelling in the cabin. The exhaust pipe job eliminated the exhaust leak, but P0420 came on soon after that. Wonder if the pipe job has anything to do with the code.

I should probably replace the O2 sensors anyway, before going any further.

I have corrected spark plug oil bath problem (along with plug replacements with NGK laser iridium) a few months before P0420 came on. No blue smoke. (I have replaced the valve cover gasket already, but that did not stop all the leak.)

One detail I neglected to mention. Right before P0420 came on, the front section the exhaust pipe, the part that is upstream of the catalytic converter, was replaced at a shop. What prompted me to do so was exhaust leak that I had been smelling in the cabin. The exhaust pipe job eliminated the exhaust leak, but P0420 came on soon after that. Wonder if the pipe job has anything to do with the code.
I should probably replace the O2 sensors anyway, before going any further.

The code has to be 90420, and you need to adress any that aren't. This is the most obvious first step here.

What about the voltage reading of the 02 sensor?