start with basics plugs wires , I've heard to check intake leaks spray carb cleaner on the engine while running and if it bubbles or sucks in ya got a problem.

 

Now that my check engine light just came on, I got an diagnostic ran. The reader gave off 2 codes of P0128, and P1810. Maybe one of those could be the reason I'm getting bad gas mileage?

 

Okay P0128 means the PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. Could indicate a faulty thermostat.

P1810 refers to your TFP Val. Position Sw (Transmission Fluid Pressure Manual Valve Position Switch). That's all I've got on that one.