What are the descriptions for

P0171 - System too Lean (Bank 1)
P0174 - System too Lean (Bank 2)

They have to do with fueling the engine. Usually when a car is running a less-than-optimal fuel mixture, it's traceable to O2 sensors, mass airflow sensor, or sometimes a coolant temperature sensor. These all have a direct effect on the fuel mixture. If your O2 sensors have been in the car for more than 60k miles, I'd change them first, as they have the biggest impact on fuel mixture. I don't know what year your car is, but if it's 95 or older, there is only one sensor per bank of cylinders. If it's 96 or newer, you'll have four - two before the catalytic converters, and two after them. The ones you'd want to change are the pre-cat O2 sensors, closest to the engine. These are used for determining fuel mixture. The post-cat O2 sensors are there for monitoring the cats' efficiency.