I have a 99 Silverado 5.3 liter V8. I'm getting these two codes, P0171 and P0174 and Service Engine Soon (SES) light on the dashboard. This seems to only happen when the engine is cold and the SES light comes on within 10 minutes of driving. After the engine is warmed up and usually after a few short trips where the engine is turned off; the SES light will go away on its own. Recently had a rough idle issue but that'* been resolved. Recently changed the EGR valve and lines, throttle control valve, all four 02 sensors and cleaned the MAF sensor and throttle body. Any clues as to what'* causing this issue? Thanks in advance!

The MAF could still be bad or you might have a vacuum leak. If the vent hose that goes into the air snorkel is broken or if the PCV valve is bad it can cause lean codes.

in addition to those above, low fuel pressure can cause lean codes from a failing regulator, clogged filter, or failing pump. the parts stores normally rent gauges free and that engine should be 58 give or take a few key on engine off and then go up into the 60'* on to run properly

Thanks, I recently changes the PCV valve as well. Inspected the vacuum hoses and didn't find anything broken or cracked. Was going to change the fuel filter but can't get it loose. It'* rusted at the connections. Even put PB Blaster on it and still won't budge. I don't want to break anything and cause more problems for myself. What about air intake manifold gaskets? Could they act this way in cold weather?

intake manifold gaskets can but usually will be just one bank. unless its the throttle body gasket. try wd40 and banging it with a hammer. just a couple solid raps nothing crazy

If the fuel filter hasn't been replaced in a while I would surly check fuel pressure like Justin suggested. If you can get your hands on a 20mm wrench because that is what the fitting is on the fuel filter. if you use a 13/16 and it is seized you probably will round the edges off of the filter. Also find a good 5/8 line wrench for the fuel line fitting.

Thanks, my next move will be checking the fuel pressure. A garage wanted $80 to replace the fuel filter....I really want to do it myself but I really don't want to strip or break something. I'm thinking it has to be something expanding or contracting because of the timing the light comes and goes off. On in cold weather, off when the engine is warm. I'll re-post when I get a fuel pressure reading. Thanks again all!

pb blast doesnt do anything really, and if you have good wrenches its not going to mess anything up. they have things to fix broke lines. just dont do it right before you have to work or something if its your only vehicle

A cold engine requires more fuel

How many times did you soak it with PB Blaster?
It worked good for mine when I changed it, and for my wheel bearing that was rusted/seized badly, I just had to soak it and let it set for a while, I think I did this three times, and eventually it worked.
Having good wrenches is the most important thing as suggested though, be sure you have the right tools.