I was driving around in my first gen s40 when my check engine light came on. I got a code reader and it came it out to be the catalytic converter code. Any help finding which parts might need to be replaced?

 

the P0420 is saying the rear o2 sensor detected too much oxygen post cat in relation to the front o2's measure. This can be caused by a failing cat, an issue with the o2 sensor(s) or an exhaust leak. Start by checking around for any exhaust leaks, particularly by the flex pipe and EGR tube. If there's nothing going on then you'd probably need to replace the cat and that's the time to also do the O2 sensors. I also believe the flex pipe is part of the cat component but your car may have two different cats. Another option is to try installing a cel-boss spacer to move the rear O2 sensor away from the exhaust stream - but that may not be "legal" in California.. Check your VIN to see if your model has california emissions or not.

 

Sometimes a long road trip will "clear" a code by burning off contaminants from inside the cat itself. "Cat cleaner" products might - or might not - help with this process, but it seems to me that it would be a good starting point, since it won't set you back big bucks. If the cat itself has a structural problem (has lost the precious metal coatings from the "grid" inside) then it's toast, though the O2 spacer trick might allow you to pass inspection (or not... don't know how stringent the CA inspectors are). But the O2 sensors DO wear out (rule of thumb seems to be over 100K miles, they're suspect) so it's usually worth a shot to just shotgun the post-cat (aka "downstream") sensor, since it's a LOT cheaper and easier than replacing the cat.