Hey guys i am new to the forum but have owned our terrain since December 2016. I have been trying to get this p0420 code to go away on our 2010 2.4 with 160k. So far I have replaced air filter, both o2 sensors with I believe Delphi, new cat from eastern catalytic, I ran crc emissions cleaner prior to replacing the cat. It currently burns about 1 quart per oil change interval the computers says. It has Mobil 1 extended in it. What is confusing about this is with brand new cat and brand new o2 sensors while reading the live data downstream sensor stays mostly between .6-.7 but still fluctuate from -.05 through 1.05 the cat gets up to 1800 under throttle. Also the st fuel trim goes from -30 to 25. It does have a slight exhaust leak in the resonator but from the research I’ve done I don’t think that would cause the o2 sensor to read like that. I appreciate any help at all. Sorry for the long post I’m not sure where to look next everything I’ve read was replacing the cat fixed it. Thanks guys !