I am getting the check engine light and the code P0420, which is defined as "Catalyst System Efficiency Below Threshold(Bank 1). I have the diagnostic report and the o2 readouts are as follows:
B1S1 Pre-Cat 845mV
B1S2 Post-Cat 825mV

I have read on here that is is usually due to a bad o2 sensor on the pre-cat side of bank 1. But with the above readings, does that make sense??? Or is it a bad cat that needs to be replaced.

I was going to try to do the o2 sensor, but I can't find anywhere on here or the web where the pre-cat side of bank 1 is and how to access it. I can see and access the other 3 o2 sensors. It appears that standing in front of the car with the hood open and looking at the engine, the cylinders are as so

^ rear of car^
Bank 1 Cat, Pre-cat o2 Sensor, mid engine bay and not accessible????
Bank 1 Cat, after-cat 02 sensor, under engine and is accesible
1 3 5
(CYLINDERS)
2 4 6
Bank 2 Cat, pre-cat 02 sensor accesible from top
Bank 2 Cat, post-cat 02 sensor accesible from bottom
FRONT BUMPER

Is this the right diagram and cylynders? I was hoping bank 1 was in front, because the o2 sensor is accessible.

Does anyone have a diagram or instruction on how to access the bank 1 o2 sensor, pre-cat side?

Thanks for the help in advance.

 

Also, this is a new engine, new muffler, previous cats with 150k miles on them, didn't have problems with the cat prior to the new engine. I light comes on..stays for a few weeks, then goes off for a while..then comes on again. I haven't noticed any difference in performance of the vehicle, no smoke or burning oil with the new engine, and the gas mileage is great, so just an annoying check engine light it seems right now, that is why I was thinking it was the o2 sensor or wiring..etc..

 

Where ever and who ever told U that a 420 code is an o2 sensor is sadly mistaken
Replace the converters.

 

I was afraid of that.

Do you recommend replacing both, or just the bank 1 converter?

Do you know or have any installation instructions if I buy the converters or should it be done by a pro?

Thanks again.

 

I would put both in because the other one will go bad shortly after.
The clamp up front is self explanatory and the ones int eh back are the same.

 

Will do,

Thanks for the help.