I have a 2001 Toyota Tacoma, 3.4 Ltr with a P0420 code when engine light popped on.?

Question

drives fine but light stays on

Answer 1

P0420 Catalyst system below efficiency (Bank 1)

This is for the catalytic convertor it is used to control harmful emission  that is emitted into the atmosphere. What causes the catalytic to degrade can be from mileage and use ,overtime. just wears out. There are many incident when it is not the catalytic convertor check the engine for a need for a tune up , do and perform a MAF (mass airflow) cleaning. this may help.