I have Chevrolet Impala 2014 model. Recently my car showing dtc code 0420. I read a article here . It describe that this is the fault related low catalyst system efficiency. I have tried every steps according the provided website but the fault code still showing.
It's possible that your catalytic converter is clogged or contaminated and no longer working properly.
But before you replace it you should check your "downstream" O2 sensor. The downstream sensor is in your exhaust system behind the converter and if it's sending bad data that can cause the ECU to flag the cat.
Another potential cause for P0420 Cat Efficiency Below Threshold (that I have personally experienced on several Hondas) is a leaking exhaust manifold.
With the engine running, check around your exhaust manifold(s) for any sign of leaking exhaust gas - there might be 2 if your Impala has a V6 (and one might be insanely hard to reach because it's likely a transverse engine).
I've had this code present for both a cracked manifold and a leaking manifold gasket.