Service function guideCatalyst Efficiency Diagnosis

This guide delivers a complete, professional workflow for diagnosing catalyst efficiency on modern gasoline vehicles, separating a genuinely failed catalytic converter from upstream oxygen sensor drift, exhaust leaks, and engine management faults. It is written for working mechanics and experienced DIYers who already own a scan tool and understand fuel trim and oxygen sensor data. You will learn how to verify monitor readiness, interpret pre- and post-catalyst oxygen sensor signals, perform a backpressure test, and confirm a catalyst replacement decision with measurable evidence. By the end you will be able to run the procedure start to finish, document your findings, and avoid the expensive mistake of replacing a healthy converter.