i have this code on my what can i do?? on 2001 Toyota Solara

okay so i been having this code for a while i change the cat the sensors and some other parts im spending a lot of money changing parts i went to 4 diffrent repair shops. and the said its the same code their dont know what else to do help

Asked by for the 2001 Toyota Solara
What code?
p0420 that code
Where did the replacement parts come from? Especially the cat.
1 more answer
If the cat is not defective or one last test could be to confirm is no false/unmetered air within the exhaust system. Best bet to have a qualified service center perform a smoke leak test while vehicle exhaust system cold. Good luck.