fallingice

Hi, I'm a new comer here. I have some problem with my car recently. My car is 2000 nissan sentra XE, the mileage is around 85000.
The check engine light came on two weeks ago. And the code is P0430. I had my car checked by one mechanic shop. And they found there were some weird green oil in one of the spark plug hole. They cleaned the hole and replaced the four sparkle plugs and cleared the code. However, after 2 days, the light got back again. So I went to dealer yesterday. The dealer said I need to replace the catalytic converter, which would cost me $600!! what I want to ask is that:
1. Need I have the engine checked completely to see if there is other problem to cause the damage of the cat. converter?
2. Before I replace the cat. converter, should I have a emission test? (I mean, if the emission test is ok, then I don't need to replace the cat. converter? will the bad cat. converter damage the engine?)

I'm a totally new driver and don't have much knowledge about the car. Thank you for your advice

Baby_Jesus

wow green stuff on sparkplugs.... mmh.

if you do need to replace the catalytic, the pre-cat may be the culprit.