Any one know why p0420 code reappear on my toyota 4runner dash board again after change my cat 2 months ago?

2 months ago the engine light is on my dash board for toyota 4runner 2005 i took to dealer they checked and told me p0420 code mean my cat ( bank 1) need to replace , they changed the cat with very costly but 2 months after it appear again and same code any one help please