BoilerUP

I have a 99 Maxima SE that is stock with about 120k on it, though the engine was replaced around 55k under warranty by Glenn Nissan in Lexington KY for faulty valve seals which had caused cylinder scoring among other issues. I recently moved to Maryland from the midwest and found out I have to get my car inspected and passed an emissions test.

I currently have P0420, P0440 and P1448 codes and an illuminated CEL.

The gas cap was replaced a year ago, so I'm pretty sure the 0440 and 1448 codes deal with the EVAP canister & purge valve. I've removed both from the car and tried cleaning the valve, but the codes are back. Local dealers want $132 for the valve and $189 for the canister, though I've found much cheaper prices online...would it be acceptable to replace only the valve and use the original charcoal canister?

As far as the P0420 goes...I've had 2 O2 sensors (of 3?) replaced on this car; 226A1-4L715 in May 03 and 226A0-4L713 in Dec 04. I've had a CEL since late 05 but never worried about it until now because the car has run 100% normally the whole time. I'm pretty confident in my ability to replace an O2 sensor but not sure how to test them to see if they're bad or not, not to mention figuring out which hasn't been replaced yet.

The local Nissan dealer wants just $95 to pull codes & diagnose, plus God knows how much for parts and labor. I'm pretty sure they'll tell me the catalytic converter needs replacing too, though I have my doubts about that. Would I be better served just spending the $$$ to get a Nissan guy to look at it, or should I start turning wrenches myself?

All comments and help are highly appreciated!!!