2007 Nissan Altima 2.5s Service Engine Soon light came on

Nissan Altima

Service Engine Soon light came on. Went into pep boys and they said I need a new catalytic converter which with parts and labor will run me around $1000. Online I can find them discounted ranging from 150 t0 400. While my mechanic says 600 for the converter. I want to buy a cheaper converter and bring it into a mechanic to install. Do you recommend that? How do I know the converter to select? Thank you so much!

Was the code P0420?
If it has 85,000 miles and in some cases under 100,000 miles the manufacturer is required to replace the converter at no charge.

You getting the part puts all warranty on you, if they provide the part it is on them. About the only difference other than price. Looking up the part by putting in the correct information is all you need. Year,make,model and engine size.