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Unfortunately code p0420 is from the cat convertor is not reducing the pollution it should out the cat convertor, and the only way to eliminate the code is to replace the cat convertor. Sorry for the bad news.
i was hoping that just the down stream o2 sensor would do it / the car belongs to my son-in law .what does the up stream o2 do?
The upstream 02 sensor is the signal to the computer to tell it if the engine is running rich or lean. The downstream 02 sensor is to tell the computer if the cat convertor is working or not to reduce emissions.
THANKS FOR HELP TODAY I WILL CHECK WITH THEM TO SEE WHAT THEY WANT TO DO . THE FIT IS NOT CHEAP
You are very welcome. Sorry about the cost.
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2003 Jetta left rear window blue plastic part of the