Do I have to replace the exhaust manifold to replace the catalytic converter? on 2002 BMW 325i

My check engine light came on and the machine at Autozone says it is the catalytic converter. When I looked it up in the manual it said to replace both. That requires more money than I have. What are my options?

Asked by for the 2002 BMW 325i
Was the code stored PO420? Low Catalytic Converter efficiency.Unfortunately if the oxygen sensors are reading correctly it is likely the cat converters are bad. Erase the codes and see if they return.