2004 Nissan Frontier catalytic converter

I have a Nissan Frontier w/ 118k miles. My check engine light came on. Brought it into the service shop for a check. Error code 0430 catalytic converter. I had them replace the converter ($750)
and the very next day the check engine light came back on. I brought it back to the service station to run another check. The same error code (0430 converter performing below thresshold) came up.
they said they will have to take it to the dealer. Any ideas as to the problem? Im broke and cant hardly afford any more repairs to the truck. Anything help you provide is greatly appreciated.
semper Fi

Tony