My 98' ES300 Got the P0420 agin and agin. Before blame the converter,
I'd like to try those O2's. My question is:

Could the O2 sensor still be bad enven if the resistance reading is in
the correct range?

I checked the resistance, they were correct. My es300 has 110k miles on it.
Dont know if the O2's have been replaced or not by the pre owner.

 

lol u dont check resistance. u check volage output

 

The Voltage should read somewhere around .45v after the sensor gets over 600 degrees celcuis. The P0420 code means the catalyst systems efficency is below the standard threshold. It could mean your converter is not performaning to the factory spec, or you also could have a short in the circuit. Get your voltage for the back o2 sensor after letting the car run for a couple minutes to make sure the o2 sensor is over 600 degrees celcuis, then get the recording, if it seems normal, you probably need a new converter.

 

also should be close to a flatline too

 

Depends, ussally the wave forms should switch from lean to rich lean to rich, and if the cat is working properly, the waveforms will be the same but it will switch from lean to rich waay slower in the rear o2 sensor then the one in the front.