The other answers are correct as far as getting codes read. The code or codes you get are are not the cures, only the places and components to find the cure. You still probably need a multi meter and a few tools. The codes tell you what circuits problems are in. There are ladder diagrams that need to be followed.

As a note, when check engine light comes on the fuel management computer has gone into default mode. It is no longer making adjustments, it goes to settings that will allow engine to run, but not at the most efficiently. Only enough to get service and not leave you stranded.

A $45.00 scan tool purchased at Autozone or Harbor Freight will tell you why the check engine light is on .

Auto Zone or Advanced Auto will scan the car for FREE!

That will tell you why the light is on.

Using too much fuel can be several things.

Here's a simple thought. Internal combustion engines need 3 things. Fuel, spark and air. Missing any of these will cause the engine not to work. Starving the engine of AIR will cause crappy fuel mileage. I'd take the air filter out and see if it needs to be replaced. A clogged air filter won't let enough air in and that will cause the car to burn more raw fuel. You can take the air filter out and drive to Advanced or the Zone, have them scan the code and they'll install the new filter at no extra charge.

Most people really do ignore their air filters.

go to any auto parts store and have the codes read.

I have a 97 Geo Prism and my check engine light came on. I got new radiator hose, new distributer cap, new plugs and wires, and new O2 sensor. I get 9 miles on a quarter tank of gas and the check engine light is still on. Sometimes the car will die when I come to a stop. What could be the problem?