On Board Diagnosis Logic

The ECM monitors the switching frequency ratio of heated oxygen sensors 1 and 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 heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst (manifold) malfunction is diagnosed.

Fig 1: Using HO2S Information To Diagnose Three Way Catalyst Function

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

Fig 2: Identifying DTC P0420 Conditions & Possible Causes

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