Question

Is a smoke test necessary to diagnose a P0420 code on a 2002 Toyota Sienna Symphony?

Answer 1

if the car had an intake leak that you could not find any other way a smoke test would be a good idea but a good mechanic could still find it even though a smoke test would help.

if the car does not have an driving issues its mostly likely a bad catalytic converter,

Answer 2

P0420 TOYOTA Code - Catalyst System Efficiency Below Threshold Bank 1
Read more: https://www.autocodes.com/p0420_toyota.html