OBD-II Error Code P0420: Catalyst Efficiency Below Threshold (Bank 1)

When the Check Engine light comes on, the error code(s) can be read from a vehicle's OBD-II interface, using a code reader; I own an AutoXray CodeScout AX700 reader (user manual). The error code experienced today is "P0420" (I just got the 420 connection, after looking at it for days).

• Error code P0420 means, "Catalyst Efficiency Below Threshold (Bank 1)". This means the onboard computer determined an error exists in the emissions system of the car.
  
  
• Each error code is made up of several sub-codes: (1) "P0": indicates the problem is in the powertrain; (2) '4': is a vehicle-specific system indicator; (3) "20": specific fault designation.
  
  
• This usually indicates one or more of three things is broken: (1) the catalytic converter, which scrubs emissions from the engine exhaust; (2) the oxygen sensor before the catalytic converter; or (3) the oxygen sensor after the catalytic converter
  
  
• There is no way to know which component is bad until a mechanic does further testing
  
  
• Buying a catalytic converter in California is a whole new world. For more information, see http://www.arb.ca.gov/msprog/aftermktcat/aftermktcat.htm