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sorry code P0420 is a cataletic convertor efficiency code, only way to get rid of it is replace that expensive convertor
DTC P0420 or P0430 Circuit Description
In order to maintain low emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the engine controls system uses a 3-way catalytic converter (TWC). The catalyst within the TWC promotes a chemical reaction, which oxidizes the HC and CO that are present in the exhaust gas. This reaction converts these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx which converts into nitrogen. The powertrain control module (PCM) monitors this process using heated oxygen sensor (HO2S) bank 1 sensor 2, which is located in the exhaust stream past the TWC. HO2S bank 1 sensor 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to effectively convert the exhaust emissions. If the catalyst is functioning correctly, the Bank 1 of the HO2S 2 signal will be far less active than the signal that is produced by Bank 1 of the HO2S 1. This indicates that the TWC oxygen storage capacity is below an acceptable threshold.
The PCM performs this diagnostic test at idle. When the conditions for running this DTC are met, the following occurs:
thats correct, normally fuel system giving vehicle too much fuel or not enough, damaged convertor, maybe because fuel pump went out, running lean for too long.