Toyota Repair: Toyota corolla 2005, toyota corolla 2005, toyota corolla

Question
Check engine light says that my catalytic converter was bad.  Did not have the money to repair at Toyota dealership, so I had repairs done at muffler/exhaust shop.  I have had front and back of muffler replaced and I replaced 02 sensor.  I am still getting a check engine light on. The dealership says that the aftermarket parts are not compatible for the car.  They want me to replace everything at dealership for 3500.00.  The only thing I haven't replaced is the Manifold, gasket and heat shield bolts, which they said I needed. Would it help to replace these items only or replace the whole thing again.

Answer
Hello, Do you have the current  diagnostic trouble code numbers available, if not please take the car to a major autoparts store like Autozone, Advance Auto, O'Reillys they help you get the diagnostic code numbers at no charge, then please reply here and I can start helping with this problem.