2005 Hyundai Elantra

Engine Performance problem
2005 Hyundai Elantra 4 cyl Two Wheel Drive Manual 52000 miles

I recently had someone use an engine scanner on my car because my engine light came on and the code that came up on the screen was P0172 system too rich. After I saw this I thought it could be that there was not enough oxygen getting into the system because my car seemed to be lagging when I press on the gas. So I replaced the air filter. I am wondering if there is some other things that I should have checked. Also when I say that the care is lagging I mean that when I press on the gas it will start to go at a slower pace but then it seems like it gets an extra dose of gas and then kicks up the speed (so to speak). Also I have noticed when switching gears that it tends to stay revved up even after I have taken my foot off the gas.
July 28, 2010.

Probable causes of an engine running rich are as follows: 1. Excessive fuel pressure can be cause by restricted fuel return line or defective fuel pressure regulator.
2. Defective Oxygen Sensors.
3. Leaking injectors and fuel pressure regulator.
4. Coolant temperature sensor out of calibration.
5. Restricted air intake system.

Jul 28, 2010.
Did you ever figure out what the problem was? How to fix it. My daughter is having this same problem. Same error code and sometime the car won't start. Anyone have any thoughts?

Oct 26, 2011.