2003 Service engine light

What needs to be replaced for code p0420?

The CAT convertor. It has less than desirable oxygen storage capacity and is failing the efficiency test. If that is the only DTC with no misfire codes, etc. then that is the most likely part causing the DTC to set. How many miles are on your Grand Prix and which model/engine size? It may still be covered under GM emissions warranty.

if there's an o2 behind the cat, couldn't that trip that code

Yes, because it's working,not because it's bad. the PCM compares the readings it receives from the pre and post cat sensors to determine cat efficiency.

p0420 can be somewhat complicated check this link for extensive info http://www.troublecodes.net/articles/catfailure/

I agree with Rich. The 02 sensor is working. Based on my experiences P0420 is never a DTC indicating the 02 sensor itself is bad.

The 2001 - 2002 Grand Prixs were delivered with known bad CATs and it is possible that some 2003 models wound up with one. If there is any emissions warranty left get it into the GM dealer before it is too late.

Again, what model and engine and how many miles?

BTW - You could remove the sensor and clean it with carb cleaner. Make sure you see light through the vanes when holding it up to a light source. Reset the DTC with a scan tool. But I'm quite sure the DTC will set again. :grinyes: