On Board Diagnosis Logic

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

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 1: Identifying Three Way Catalyst (Manifold)

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

Fig 2: DTC P0420, P0430 Three Way Catalyst Function - On Board Diagnosis Logic

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