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Old 05-28-2015, 09:24 AM   #2 (permalink)
2000mc
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A dirty post cat O2 sensor should be a very unlikely cause of a p0420, in fact it could even prevent that particular code from setting. A p0420 sets because the post cat O2 is seeing the simlar switching from lean to rich, that would be the normal signal from the pre cat O2. If the cat is working normally the post cat O2 produces a near flat signal, steady / steadier voltage
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Eddie25 (05-28-2015)