P0420 on 2001 Toyota Corolla

Answer 1

Catalyst system efficiency below threshold(bank 1). Check for loose hoses(vacuum leak) intake and exhaust manifold(s) for air leaks and finally the catalytic convertor may need to be replaced. If the catalytic convertor needs to be replced check to see if it has any warranty remaining this is covered by Federal Government Regulations and comes usually with an 8years or 80000mile warranty.