On Board Diagnosis Logic

The ECM monitors the switching frequency ratio of air fuel ratio (A/F) sensor 1 and heated oxygen sensor 2.

Fig 1: Identifying Exhaust System Components

Courtesy of NISSAN MOTOR CO., U.S.A.

A three way catalyst (manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase.

When the frequency ratio of air fuel ratio (A/F) sensor 1 and heated oxygen sensor 2 approaches a specified limit value, the three way catalyst (manifold) malfunction is diagnosed.

Fig 2: DTC P0420: On Board Diagnosis Logic

Courtesy of NISSAN MOTOR CO., U.S.A.