What does engine code 420 mean on a 2003 Ford Focus?

Answer 1

code P0420 - Catalyst System Efficiency below Threshold

what it means is that the car thinks the Cat is malfunctioning or broken. Sometimes this pops up if an aftermarket cat is installed or the cat is removed entirely when people are tuning their car. Often there are solutions to "turn off" or cancel the downstream O2 sensor.

If you haven't altered your car lately don't be so hasty to just replace the downstream O2 sensor or the cat. The code comes up because the upstream and downstream O2 sensors are too similar. This could be caused by one of the two O2 sensors malfunctioning. This could also be caused by the engine itself running too rich or lean. There also could be an exhaust leak somewhere letting air in.

Here is a website for help:

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