The Engine Control Module (ECM) observes the switching frequency ratio of the front and rear oxygen (O2) sensors. The Three-Way Catalyst (TWC) can store oxygen and will indicate a low oxygen sensor 2 switching frequency.
When the oxygen storage capacity is reduced, the switching frequency of the O2 (Oxygen Sensor) will increase. If the frequency ratio of the oxygen sensors reaches the specified limit value, the fault code P0421 OBD2 is set, indicating a malfunction of the catalytic converter (TWC).