Most common reasons why "Check Engine" light is on:
- O2 sensor (or Lambda sensor which is monitoring the air/fuel mixture) - you should try to replace this one first
- Fuel quality or fuel filter - if it's not in the fuel tank, you could try to replace this one also
- Spark plugs (could be wires also)
However, it's worth checking it on tester to be sure what the problem is before replacing half of the car. The three points mentioned above shouldn't cost you much.
Hope it helps