Hey, I have a 2018 NX 300 model, I’ve had it for the past month or so, sometimes suddenly when I’m driving The check engine lights up the Code that shows up after using my OBD reader is P042000 which states ( catalyst system efficiency below threshold bank one) . I tried running some live test on my sensors and based on my minimum knowledge, I checked bank one sensor one voltage which was fluctuating around 3.6 to 2.5 volts and my oxygen sensor, which is bank one sensor two was fluctuating around 0.88 and 0.65 volts

at first, I thought the catalyst converter needs changing, but then I thought about could my engine be running Rich or lean .
that’s when I thought about checking, short-term and long-term fuel trims, short FT B1S1 fluctuate between 5.4 and -0.15 but the long-term was steady at -12 which to my knowledge indicates a rich running engine. ( all numbers were taken while my car was idling at normal engine temperature )
to be honest, I don’t really know what to do right now and I’m writing this post for you guys to help me complete my diagnosis and solve this problem
Thank you in advance

Edit : my car mileage is around 30k miles ( thought it could be helpful to say )

 

Check air cleaner and air sensor is both clean.
Go somewhere safe and drive the car so the engine has to rev and pull very hard like Fangio.
ie a bit of a thrash.
Then see if anything changes .