First step - have the stored fault codes or check engine light scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA.
Then, post a reply as 'add answer' with the specific code for more assistance.
This aspect is important; there may be a fault code stored in the OBD memory which may not be displayed as a check engine light.
Do not skip this important first step - there are literally hundreds of possible fault codes. Avoid wasting/ throwing money at the problem.
Vacuum Leaks- One of the most common cause for the P0172 code to show is a vacuum leak. Take a look around and see if you can find the vacuum leak.
Oxygen Sensor- The P0172 codes is thrown when the oxygen sensor(s) do not register the right amount of oxygen in the exhaust. When the oxygen sensor is bad, it can cause a false reading and trigger the code. The computer will still adjust the air/fuel mixture to compensate.
Mass Air Flow Sensor- The mass air flow sensor calculates the volume of air entering the engine. If it has gone bad, the air fuel mixture will be off and the P0172 code will show.
Bad Fuel Injectors- If you have one or more fuel injectors that have gone bad that can certainly cause the air/fuel mixture to go bad.
Fuel Pump/Fuel Filter- Today's modern fuel injected motors need a lot of fuel pressure to atomize the gas properly. If the fuel pressure is off, atomization doesn't happen properly and the optimal air/fuel mixture isn't achieved.
Air Filter- While it's not the most common reason to throw the P0172 code, a clogged air filter certainly can cause the code to throw. And, since it's so easy to check, why not look?