Catalyst Efficiency Monitoring

It compares sensors before and after the catalyst. Codes such as P0420 point to low efficiency. It is one of the OBD readiness monitors. Replacement catalysts must be approved for the vehicle, and cheap universal parts often fail the efficiency test again. Before replacing the catalyst, rule out exhaust leaks, a faulty rear sensor and engine problems such as misfires or oil burning, which damage new catalysts too.

A working catalyst stores oxygen and smooths the rear sensor signal.

Exhaust leaks or sensor faults can mimic a bad catalyst. Misfires can damage the catalyst.