P0420 and P0430 are for Three Way Catalyst Functions

The ECM (Computer) monitors the switching frequency ratio of tront and rear oxigen sensors. A three way catalyst with hight oxygen storage capacity will indicate a low switching frequency of rear oxygen sonsor. As oxygen storage capacity decreaese, the rear heated oxygen sonsor switching frequency will increase.
When the frequency ratio of front and rear oxigen sensors approaches a specified limit value, the second stage diagnosis is pplied.
Thes econd stage diagnosis switches the maixtue ratio feedback control using front heated oxigen sensor to rear heated oxigen sosnor.
Then the ECM measures the switching lag time between front and rear oxygen sensors.
The longer lag time indicates the greater oxygen storage capacity. if the lag time is within the specified level, the three way catalyst malfunctions is diagnosed.

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