Car Repair > error code of P0450 > For Questions

Hi...
I bought a 2001 manual Nissan Sentra XE with 75K miles on it a couple weeks ago, and I'm blind about cars mechanically.
The record was clean, and I did a courtesy inspection through Nissan dealer, and everything was fine except a minor leaking from the transmision(the cap I believe so)
Since 5 days ago the Service Engine Soon light stays on, I brought it to autozone and they detected my car with an OBD II, and it gave an error code of P0450.
I checked the gasoline cap and it is properly closed.
For your information, the last time I put the gasoline was a couple days before the SES light turns on. The low-on-gasoline yellow light was on and I immediately put the gas in the tank around 7 gallons.

The guy from autozone says that it's probably some unused gasoline that clog inside/something like that, and he said it won't do anything harm to the rest of the car engine/transmission/machine besides I just won't pass the emission test whenever I do the registration with DMV; is that true?
If it won't harm anything, how can I turn off the SES light?
If I need to take it to the dealer/auto shop, how much is it going to be to fix this?What is exactly the name of the parts that I have to change?
I'm a student so I'm really tight on budget right now, so I'll do things by my own if it's possible

I went back to the car's record and the previous owner just passed the emission test on March this year. According to her and some of the paperworks shown, she changed the oil regularly(every 3000 miles) and do any other necessary routine change/repair.

What could possibly wrong with my car?

Thank you very much for taking your time reading this :)

Chris

Optional Information:
2001 Nissan manual XE 1.8L

Already Tried:
tighten the gas cap