I'm assuming Advance just hooked an OBD2 scanner up to it and read the codes?

It's a hard thing to narrow down sometimes. The ECU compares the front and rear O2 sensors, which are obviously on each side of the cat, and uses the readings to check if the cat is within proper operating spec. One of the O2 sensors is a wideband unit usually, and one is a cheaper narrowband. The wideband is usually used for most engine operation, and if that O2 sensor goes bad you will generally have problems with the engine running well. So if the engine is running fine, and you're getting a cat efficiency CEL or whatever, replace the narrowband (rear I'm pretty sure, but check just in case) first because it's cheaper. I don't generally like to recommend blindly replacing parts to try to solve a problem, but O2 sensors aren't something that can be easily tested because of the heating elements and the fact that they frequently fail slightly (as opposed to completely).