Hi,
So I have Chevy Cruze 2011, 176k miles, automatic, 1.4l turbo . Initially I got it in poor condition to play with. Currently there are brand new : Intake manifold, valve cover, fuel injectors, turbocharger, spark plugs, ingition coil, evap purge valve, evap solinoid and canister, both O2 sensors and catalytic converter (as old was popping P0420 too). Checked for vacuum leaks and all is good. Now car drives very well, has lot of power, passes emission tests , but sometimes it may get P0420 code. This happens only when car is stopped after some short drive (like 3-5 minutes, then stopped on red light and then check engine goes on). Today that happened again and what cut my eye in freeze frame is that it showed engine running in open loop due to insufficient engine temperature, while registered temperature is 203 degree, how is that possible ? Well, Cat is not expected to work correct in open loop, so that feels like a reason, but how it may go to open loop when real temperature is in working range? Catalyst temperature was registered above 1000 degrees...
Thanks !
Here are screens of freeze frame
https://postimg.cc/gallery/nsQPz8p
With all the new stuff and still seeing a code makes me suspect your temperature sending unit sending wrong signal to ECU. Maybe pull it and check it for resistance or just replace. They're fairly cheap and yours is probably the original, 11 years old.
It is new too, forgot to mention, replaced it with water outlet together... I will try to capture graph for temperature, maybe there are some short drop outs...