P0420 code

Yesterday I filled up at a gas station that I rarely go to. Five minutes later check engine light was on and P0420 code. Is this usually the front cat/O2 sensor, or the back cat/O2 sensor? I do have a coolant leak near the head gasket, not sure if that is an issue here too.

 

judging from it happening right after you filled up at an unfamiliar place it...
could be bad gas

it could be that you didn't tighten your gas cap all the way down; causing a vacuum issue

OR... it could be 9,264 other things

check out this thread
http://www.tacomaworld.com/forum/1st-gen-tacomas/195841-need-help-check-engine-light-code-p0420.html

 

The code came back. I tested the rear O2 sensor resistance and it checked out. The front one had nothing. FSM said at cold temperature around 1-2 ohms. My meter didn't get anything. That sensor is a Bosch from years ago when I replaced it. Is it possible this is my code 0420 culprit?