Outline Of Diagnosis

Check the catalyst efficiency deterioration.

Generally, the catalytic converter exhaust gas conversion efficiency and oxygen absorbing/removing function are inter-related. The front A/F sensor and rear oxygen sensor are therefore utilized to monitor the oxygen absorbing/removing function, and the results are used to monitor the conversion efficiency.

Fig 1: Identifying Catalyst Efficiency Deterioration

Courtesy of SUBARU OF AMERICA, INC.

Oil adherence to the catalytic metal (an alloy of platinum and rhodium, or an alloy of platinum, rhodium and palladium) in catalyst converter or metal melting by extraordinary high temperature result in the decrease of metal surface area contacting exhaust gas. Eventually, catalytic conversion efficiency blunted and oxygen density of catalyst downstream may change.

Fig 2: Identifying Oil Adherence To Catalytic Metal

Courtesy of SUBARU OF AMERICA, INC.

Output of rear oxygen sensor changes gradually with a new catalyst, but the blunted catalyst efficiency mentioned above makes sensor output larger and inverse time shorter.

Therefore, catalyst diagnosis is conducted by monitoring the rear oxygen sensor output and comparing its output with the front A/F sensor output.

Fig 3: Rear Oxygen Sensor Output Blinking Patterns

Courtesy of SUBARU OF AMERICA, INC.