2005 Trailblazer LT I have a P0420 code. Graphed both o2 sensors and they both read the same. Thought i had a bad upstream sensor. Changed it and i still have the code. I'll post the graphs of it currently. One is just at idle, the other is approximately 3000 rpm for a few seconds. They are different but I still get the code. I have awful gas milage but i don't seem to have a loss in power. The previous owner drove around for monthes with a bad thermostat before i got it. I'm assuming its a bad cat. Thoughts/opinions. The purple line is the upstream.