White smoke is likely an indication of head gasket failure and that could have contaminated your catalytic converter or O2 sensors. While your converter could be shot there are other possible causes for P0420.

https://www.walkerexhaust.com/support/tech-tips/diagnosing-catalytic-converter-p0420-code.html

https://www.obd-codes.com/p0420

However first you should probably check for head gasket failure.

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White smoke is usually a blown head gasket, unless it's just condensation from a cold start in cold weather. (That would go away when the engine warms up.)

Since the check engine light is on what came up when you scanned for codes?

Your symptoms are definitely indicative of a blown head gasket. Use a head gasket tester that you can rent at AutoZone, and follow these steps: https://youtu.be/v0U8Iyf0kk4?si=nWj9IArjLfMl9LLE

If the solution turns yellowish from blue, the head gasket is blown. 

I have a 1998 honda civic with the engine 16Dy7 and auto trans. The check engine light came on and also white smoke started coming out from exhaust and the catalytic on the manifold. The code that shows in the scanner is P0420 and it says Catalyst system efficiency below threshold bank. Is the Catalytic converter Shot?