P0420 error, new cat. converter or sensor?

[200CorollaS2001]

I have a 2001 Corolla S with 111k miles on it. Recently, I had the engine light come on with P0420 indicating emissions weren't within threshold.

I've heard most of the time it's the catalytic converter, and I've heard it might be the second O2 sensor. From what I've read on these forums, I tend to believe it's the converter. Plus, I have been thinking the performance seems a little less than it used to be.

Odd side note, a coworker also has a 2001 Corolla CE with 99k Miles that got same error same time. And we both use same gas station.

Any advice would be appreciated. And yes, I just noticed I misstyped my forum name, lol. Oh well.

[fredk]

both

[ShawnM]

Yea....what he said. With 111K on it your better off replacing both if you havent done the O2 sensors yet, though it is most likely the converter that is causing the code. Just out of curiousity, what brand and grade do gas do you use?

[200CorollaS2001]

Cheveron 87 octane is what I try to keep it to. Secondary goes to Shell, Exxon and Amaco.

I am the 2nd owner, the first one defaulted on payment, no accident records. I bought it with 62k miles on it, and the oil was clean synthetic with a premium filter. I've kept it with Mobile One 5w-30 sythetic and premium filters. Oil consumption is around 3/4 qrt around every 1500 miles very roughly.

It's been a perfect car all the way around, just standard maintenance, brakes, oil, belt and tensioner pulley change. I did the plugs a while back with OEM irdium plugs, noticed that previous owner had already changed them.