Service Engine Soon light is on. Code shows P0420

Question

Service Engine Soon light is on. Previous test shows code P0420

Answer 1

P0420 usually 90% of the time means the catalytic converter is bad, if there are no exhaust leaks 

P0420 is bank 1 

In some cases these might work if the catalytic converter  is not already overheated or damaged. Use 91 octane from Shell along with a good fuel system cleaner like 3M or BG 44k and maybe do any intake flush to decarb the engine.