I have a 2010 Buick Lacrosse that had the P0420 code. Replaced the Bank One Cat and the code and the check engine light came on again with the same code.

Did they use an original equipment cat from the dealer or an aftermarket one? Aftermarket cats can cause more problems than they fix. There very few that I will personally install.

are they monitoring downstream O2 sensor data to see if the O2 sensor may be biased high? If the cat is doing its job the downstream O2 sensors will read a lower voltage. Tricking the vehicle into running rich or lean will cause this voltage to change ( unplugging a coil will cause the Downstream O2 to read high as the cat tries to burn off the extra fuel. Unplugging a fuel injector will cause the downstream O2 sensor to read low as extra air is being pushed thought the cats). Making sure the downstream O2 sensors are capable of switching should be part of the diagnostics.

Are you sure they replace the right cat? There are 5 different engine options for this car. If you let me know what engine you have I can let you know what bank is bank 1.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

I have a cxl 3.0.... Where is bank 1