2004 Volkswagen Passat PO421 Bank 1

A code P0421 may mean that one or more of the following has happened:

The catalytic converter is no longer functioning properly
An oxygen sensor is not reading (functioning) properly
A sparkplug is fouled up

You might want to have the exhaust gases checked before and after the catalytic converter to rule out a defective converter-inspect spark plugs-measure the oxygen sensors switching activity

Possible Solutions Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into

Have the oxygen sensor checked and tested its sensing that the ctatalytic converter is malfunctioning-