P0171 and P0174 codes are for lean conditions on both banks of the engine. This means the O2 sensors see to much air in the air/fuel mixture.
As stated above. This means air is entering the system someplace, or there isn't enough fuel.
This leaves a lot of options, however, since the codes are for both banks of the engine that tells me it's not a bad O2 sensor or some type of intake manifold leak.
You likely have a bad mass air flow sensor. When this sensor fails, the computer will try to add fuel to compensate, when it can't, it will throw the lean engine codes that you have.
I would not be doing my job however if I didn't mention you could get these same codes from a bad fuel pump. As strange as it might seem, this is why mechanics get paid what they do, these all have to be tested. Now here is a list of the possible causes for your codes.
Possible causes Intake air leaks Faulty front heated oxygen sensor Ignition misfiring Faulty fuel injectors Exhaust gas leaks Incorrect fuel pressure Lack of fuel Faulty Mass Air Flow (MAF) sensor Incorrect Positive Crankcase Ventilation (PCV) hose connection
I hope that you are able to get this resolved, I believe you need a new maf sensor. As for the other code, I wouldn't be to concerned since it's gone.
I hope this info helps you out.