Diagnostic trouble code P0420 indicates a catalyst efficiency problem.

The OBD2 system uses oxygen sensor readings before and after the catalytic converter to measure catalyst efficiency. 

Efficiency is a performance measure of the catalytic converter.  An efficient converter can store the amount of oxygen needed to perform its catalyzing functions.

When catalytic performance drops below a certain level, code P0420 will set and the malfunction indicator light (MIL) will illuminate.

Some common causes for this code include:

Defective catalytic converter, restricted exhaust, exhaust leaks, lazy oxygen sensors.

Tips:

Check for technical service bulletins before replacing components.  Some manufacturers have PCM recalibration (software updates).  Some models may also have extended warranties to cover problem converters. 

When replacing catalytic converters, make sure the new converters are certified to do the job.  Too many times, smaller, aftermarket converters are used which may not satisfy the O.E. vehicle requirements, causing a return of code P0420, along with some unneeded aggravation.

Root causes:

If converter replacement is necessary, don't forget to address the problems that contributed to damage of the original converter.  A rich running or misfiring engine is probably the number once cause of converter damage.

Did you know?

Did you know that motor oil can cause catalytic converter damage?  It can if it's blended with too much of the anti-wear additive, zinc dialkyldithiophosphate.