Looking for advice as I clear this code and it will come back after a month or so on my v6 2005 runner.

P0420 $07E8 1/1
Catalyst System
Efficiency Below
Threshold Bank 1

From what I have read there are several O2 sensors. I believe 4? Two before the two cats (Canadian vehicle) and two after the cats on both the driver and passenger sides.
I believe the O2 sensor before the cat on the driver side is firing this code?
I read that diagnosing this issue is a bit tricky and that knowing the milage may help. I have 110,000 miles on the runner right now and the first time it fired was at about 80,000 miles.

The truck runs fine and I haven't noticed any issues like awful fuel milage and rough running.

Do you think I should replace that O2 sensor and see what happens ?

The O2 sensor could be bad but since a lot of things can generate those codes I would test it first. Could just be a loose connection causing the heater to cut out or a bad unit. You can check the voltage going to it and measure it's heating coil for continuity and check the output, which should be approx. 0.5 volt after it's completely warmed up, with a DVM with the engine running at the correct fuel mixture. You can also take it off and test it on the bench using propane (Google it) or try swapping in a good one that you can get on Amazon for around $60 if you want to make absolutely sure it's really the problem.

Thanks for the reply Steve.

AllparForums: Testing o2 sensors with a dvm - Minivans · Pacifica - Allpar, the Chrysler Car Community

I found this post and thinking of performing this test. What O2 sensor is the one faulting, is it the one before the cat on the drivers side ?
What color wire is the O2 signal wire?