2001 Toyota Corolla - Engine Code P0420

The check engine light came on this morning so I went to Advanced Auto to use their engine code reader. The code is P0420 and says “Catalyst efficiency below threshold”

Does anyone know what this means and it’s a DIY job?

This code generally means the catalytic converter isn’t doing it’s job. However, this may be a fault with the downstream O2 sensor that is monitoring the cat. This car may still fall under the Federally-mandated 8 year warranty for emissions equipment. Take it to the dealer, and let them know about the warranty.

The code P0420, from the engine computer, means, “the catalyst SYSTEM efficiency is below the threshold [of the allowed efficiency for emissions control]”. In other words, the catalytic converter is REPORTED as not performing its job as well as it should.
The engine computer compares the REPORTS from the front oxygen sensor and the rear oxygen sensor, and compares them with each other and with its built-in values. If those REPORTED values are not right (in its judgement) a code will be set and the check engine light will be turned on.
You can change all the parts, if you want to; or, you can do the checks on the parts by following the instructions in the repair manual. You may find a mechanic who can do these checks. Ask around.
FYI: http://www.obd-codes.com/p0420