Fuel grade/ethanol contribute to cat error?

Hi there,

I'm new to this forum. I just bought a 2002 Tacoma SR5 TRD 4x4 with 180k miles.
I just filled the tank, the first complete fill-up, and the next day (today) I noticed the check engine light was on. I immediately thought I forgot to replace the cap or something, but no. I brought it to get scanned and came back with code p0420: Catalyst system efficiency below threshold (bank 1). I see the recommended repair is to change the O2 sensor (the first one at least). I am hesitant to do so, for the cost, because I was considering an exhaust replacement and the $120 or so could go towards that.
Now, in reading here I have seen that my truck may be sensitive to high ethanol in lower grade fuel. Causing the sensor or cat to get "confused" by the extra ethanol. Could this be the reason my check engine light came on? If so I guess I need to just run out the tank and fill with mid-grade or premium. I did think however that maybe an ethanol fighting additive would help to offset the problem. Any thoughts?

Thank you.

PS I am posting some other concerns under diffent topics/titles, so please check them out! Thanks.