exactly
if its a 2007 or later there should be 2 of them (on four cylinder engines)

the second is to check the performance of the cat

the whistling was probably either that belt (did the sound change after he changed it?)
or
the exhaust going past the partially blocked muffler

if the pedal was set improperly the machine would have warned you

now if he told you the cat never reached operating temperature i wonder how it could if it was full of condensation keeping it cool?

check the new 1 see if it has a drain hole in the bottom of it

that would mean that this is a know issue and the original was replaced with a new design to deal with it

where in NY are you im on LI maybe i can help