I took the car to auto zone and they put the code reader on the car...I have two codes - P0171 and P0174 saying something about lean?

The thing is when I start the car it splutters for a few seconds and then settles down to a normal idle speed...about 800K....I took it for a nice drive today and it ran smoothly so looking at some of the things that could have caused the CEL to come on would the car run that well if it was something like a sensor or bad vacuum tube? I did get caught low on gas a couple of weeks ago...down to less than 20 miles..

Thanks

Gary