Diag. Code PO420; PO115 2002 Camry

2002 TOYOTA CAMRY 4 CYLINDER 2.4 LITER
Mileage: 115k
PO115:Coolant Temp Sensor Circuit
PO420:no clue, something to do with catalytic converter, general code for bad o2 emissions or something.
Don't know if it's a non-PZEV emissions engine

I'm having a few physical and intellectual dillemna's here.
I'm getting these 2 check engine codes on my girlfriend's car. The 115 is 420 have been there since January 2007. And i'm just now getting to fixing them. I should mention that the only symptoms I can actually "see" is that the car gets reall, really bad gas mileage. Consumer reports tagged it at 25 miles per gallon which is horrible in itself, but I think I get even less now. The car previously had a badly damaged Serpentine Belt, though I was able to replace that, and since then i've "restarted" the check engine light by taking the negative off the battery for a few minutes.

I've researched and been told a few things and I want some opinions about them.
PO115- replace the sensory, costs around bucks(true) and is located in cyclinder though i've yet to find. Help would be appreciated
PO420- What the hell is this? Lol I've been told to replace my manifold assembly, though which one do I replace. Will that even fix that problem? What about the catalytic converter or my exhaust manifold. Or is that what was meant by "manifold assembly"?
Also, this car is 5 years old however when she bought it, it had somewhere around 70k miles. She's since moved across country(vegas -> Florida) and doesn't know how to converse gas worth a cow's patty. Is this what I should expect from now on? I mean is there anything that a simple guy(23 yr old) can do to maintain a car or is it inevitable to fork out my paycheck and some more to fix what really are minor issues when it comes to very complicated piece of machinery. I'm willing to fix anything myself but the further into "auto repair" I get the more pricey it is to learn to fix and buy the tools and parts to repair.