On Board Diagnosis Logic

The ECM monitors the switching frequency ratio of heated oxygen sensors 1 and 2.

Fig 1: Diagnosis Procedure DTC P0420, P0430 - Description Of Three Way Catalyst

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

A three way catalyst 1 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 1 malfunction is diagnosed.

Fig 2: Diagnosis Procedure DTC P0420, P0430 - Possible Cause Table

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