replaced cats - now new code

[Bowed]

2001 silverado 1500 - 4.3 v6 - 4l60 - 155k miles .


I was getting several codes that pointed to bad cats . I replaced the assembly with a factory complete Fischer cat assembly rated to be OE manfacturer quality . I cleared the codes and then drove the truck for a day (25 miles ) without any problems . The next time the truck went out it set code P0300 . What are the chances the cats are not doing the job , or is P0300 usually something else ? Honestly at this point I am clueless to what is going on with this truck . Thanks for any input .