Q: Q: P0420 code

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I noticed a few months ago my check engine light would go on and off. Now it stays on and I was told it won't pass smog. The code is p0420 "b1 cat system system below threshold."

I am curious to see if I can fix this problem or pass smog in ca without replacing. Also my spark plugs are in need of replacing I believe could that affect the codes? What would be your course of action if you need to pass smog with this code

My car has 171000 miles.
My car has an automatic transmission.

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