Nissan Repair: service engine soon light, nissan dealership, gas mileage

Question
We have a 2001 maxima GLC with 25,000 miles.  The car runs fine and gets good gas mileage. A couple of days ago I left the gas cap off for a short time and the service engine soon light went on.  This happened once before and after replacing the cap I figured the light would go off.  It didn't and is still on after two weeks. I checked the air filter and replaced it.  The light still did not go off.  My question is what other thing might cause this problem and how serious might it be.  How soon should I have somebody look at it?  Also do you think that any garage would be able to handle this or need I take it to a Nissan dealership.  

Thanks for time.

Dave Haas  

Answer
Dave,

The problem was with the evaporative control system on the gas tank.  The fuel system is closed and vents through a cannister that is under the car by the gas tank.  Well, when the cap is loose or off it will set a code.  I would take it to Autozone or Checker and see if they will read the code for you and when you explain that you had left the gas cap off a couple of weeks back they should reset the light for free.  If not you can purchase a code scanner that you can read the codes as well as clear them for about $65 bucks.  So, it would be cheaper to do it youself if the stores will not.  Also, the connector for the On Board Diagnostics is under the dash on the driver side.