2003 Impala 3.4 ltr code p0420a

Hellow everyone. I'm. working on this car it has 187000 km it is hard on fuel does not seem to have the power it should have. Could somebody please tell me what could trigger this code. thank you for all the help.

This is a catalyst efficieny code. The sluggishness that you describe is due to a restriction on one of the catalytic converters. This puts more stress on the engine and can cause more problems the longer that this vehicle is run like this. To best check this is remove the O2 sensors and more than likely it will run better since it has less restrictions.