Aftermarket catalytic converters usually contain a lot less precious metals than OEM. This means that their value will be low.
There are some signs to look for in order to recognize an aftermarket converter:
● They may have a silver shield on them.
● Arrow on the shield.
● Many codes will begin with N. for example NCE, NAT, NT, NGE.
● Aftermarket cats are usually lighter.
Here are some examples: