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the 420 is a cat code. the cat converter needs to be replaced. the precious metals are worn out and the converter is not doing its job anymore
171 is a lean code. possible causes are vacuum leak, mass air flow flow sensor, pcv valve or low fuel pressure from a bad fuel pump.
the 170 code is related to the 171 for fuel trim.
anyway to test convertor
you need a real scan tool to monitor the O2 sensors. comparing the 2 wave patterns will confirm. the fact that the code set is a 99.9% chance the cat is bad
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