HondaCivicForum.com - Got a question about p0420 (and a funny story, maybe)

Hi, this is my first post, I know, I know .. use the search function . Although, I can't seem to find an answer that satisfies me .

On a side note, i have p0420 on my wrx, but i know it's probably something to do with headers/full turboback catless exhaust .

Here's a little background info on this 2000 Honda Civic my girlfriend owns . So far this car has been a nightmare :)

She bought is used at 70k from the dealer as a certified pre-owned car . She has been doing oil changes, but that's about it . Aside from when her car died on the side of the road and the car got it's (i assume first) ignition coil replaced by some sheisty philadelphia shop (unfortunately, i have no record of those clowns, including their name) . But anyways they replaced her coil for a mere 350$ and in the process snapped her dipstick and sent the metal rod sailing into the oil pan . She was sent on her way without being told . And found out after being refused oil changes .

I came into the picture after this and noticed that the dipstick tube was leaking oil ... lots of it . She had no way to check, but added a quart every time the light came on (this went on for maybe 1 or 2 months, I'm uncertain exactly).

After replacing the dipstick tube, and doing the first brake job ever on this car .. yep .. honda certified .. never had brake shoes or pads done .. she was right up against the metal backings of the pads and shoes . So that required new rotors, drums, and a bunch of fun stuff (did I mention that the wheel cylinder was missing the springs inside it on one wheel ?)

Anyways, after changing her brakes . She started getting a flashing MIL, misfires . (unrelated i know, but hey ... whatever, you catch flack sometimes) . To where it was noticed that the ignition wires were OEM and appeared to be original, same goes for spark plugs which were fouled beyond belief., and a sediment covered cap/rotor . .. that's standard stuff to replace with a coil, no ?!

After this all was replaced she pops up with a p0420 .

Now, I'm sitting here not wanting to put any more of her (OR MY !) money into this car . I am getting mixed results on a search as to what the issue can be . According to what I'm reading it could be one of three things, Upstream O2, Downstream O2, or the cat itself .

I want to assume an O2 because of the misfires, not to mention the expense of a cat .

So here is my question (after this little novel)

Is there anyway I can test the O2 sensors and the cat ? So I know which to waste more money on ?

Thanks !