The catalyst efficiency system is a three-way catalytic converter that helps reduce emissions by oxidizing or reducing the pollutants in a vehicle's exhaust until they are less toxic. Other components of the exhaust system, such as the O2 sensor and the exhaust pipe, are commonly considered part of the catalyst efficiency system as well.
Catalyst efficiency system problems are often overlooked until an emissions inspection is failed or the P0420 error code comes up during a mechanic's diagnostic check. The typical problem is that the catalytic converter is clogged or malfunctioning, but the error code could also indicate that the O2 sensor's wiring or connectors have been damaged, the exhaust pipe has been harmed, or exhaust leaks are present.
To diagnose the exact problem, first look at the entire exhaust system. Note any holes, dents or unnatural discoloration. Listen for any rattling sounds coming from the system. Any of these symptoms point to a need to replace the catalytic converter. If those symptoms are not present, move to the O2 sensor and check for any visual damage, such as broken wires. If the sensor looks undamaged, use a scan tool or oscilloscope to test it further; if it releases anything other than a steady waveform, the sensor needs to be replaced.