Can't get check engine light to stay off.

when I check the system P0420 comes up saying catalyst system, what does it mean and how can I fix it

 

How long has it been since you replaced the Oxygen sensors (aka O2 sensors)?

There are upstream and downstream sensors meaning one located before the cat converter and one located after the cat converter. The O2 sensor above is used to adjust the fuel tables in the engine's computer to provide the proper amount of fuel to the engine. Now the O2 below the cat converter is used to measure how well the converter is converting the exhaust gases to less harmful gases. If the bottom O2 sensor is wore out then it can supply bad information to the computer even if the cat converter is working properly. Also, if both the top and bottom O2 sensors are wore out then you'll get a double whammy of lower gas mileage and the check engine light due to too much fuel being fed to the engine which the cat converter cannot handle (overly rich).

Solution? Replace both O2 sensors at 100,000 miles.
Upstream
Buy Bosch Premium Oxygen Sensor 16291 at Advance Auto Parts
Downstream
Buy Bosch Premium Oxygen Sensor 16292 at Advance Auto Parts

 

Good thing I found this thread. Im about to pickup a 01 S4 tonight and from what I heard it needs one of the 2 sensors replaced.

Thanks for the Advance links.

 

If it's the bottom one and a new O2 sensor does not resolve the issue then try this:
Oxygen Sensor Spacer (CEL Boss)

Fixed a recurring "cat conv below efficiency" check engine light.