A few days ago, the check engine light came on while I was on the highway. I scanned it and it gave me a P0421 code, which I understand is related to the cat.

The car seems to drive fine, but I'm uneasy to be driving with the light on.

OBD-codes.com says that
* The catalytic converter is no longer functioning properly
* An oxygen sensor is not reading (functioning) properly
* A sparkplug is fouled up

What can I do to narrow the problem down?

Save, clear and monitor farther ....

I would clear the trouble code and keep driving. If the same code P0421 comes back, you need to replace the left cat.

How many miles on the car?

How do you clear the code?

Torch the car.

Get a VAGCom !! stop screwing with OBD-- the free be vag program and generic cable from Ebay seem to work OK - and you can even clear the code.