P0420 = catalyst system efficiency below threshold (bank 1)
the computer is detecting that the O2 sensor downstream of the catalytic converter on the bank of cylinders that includes cylinder 1 is starting to mirror the same cross counts as the upstream O2 sensor. This usually means that the catalytic converter is worn out and must be replaced. However, the oxygen sensors must check good---a bad or "lazy" oxygen sensor won't give an accurate picture of catalytic efficiency.
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