Check for diagnostic trouble codes with an OBD-II scanner
Inspect for rattling noises from the converter
Compare inlet and outlet temperatures with an infrared thermometer
Measure exhaust backpressure with a pressure gauge
Perform an oxygen sensor live-data test
Check for sulfur or rotten egg smell from the exhaust
Inspect for visible damage, dents, or discoloration
Test engine performance for loss of power or poor acceleration
Verify exhaust flow for restriction
Compare upstream and downstream oxygen sensor readings
Perform a vacuum test for exhaust restriction
Check emissions test results if available