On Board Diagnosis Logic

The ECM monitors the switching frequency ratio of air fuel ratio (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: Checking Three Way Catalyst (Manifold) For Exhaust Gas Leaks

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

Fig 2: DTC P0420 Diagnostic Chart

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