Code p0171 means that the computer has noticed (by reading the oxygen sensors) that there is too much air going into bank one (the bank that includes cylinder number one in the firing order in v style engines). This information is also applicable to code p0174 as this code is the same except of the opposite bank of cylinders.
Engine control computers need to have a specific ratio of air to fuel in order to work properly, and if there is too much of one or the other then it will set a code.
There are several very common things that cause this code:
Dirty Mass Airflow SensorVacuum LeakBad Oxygen Sensor(s)Running Out of Gas
Some other things that can cause this code:
Clogged Fuel InjectorsLeaking Exhaust SystemEvaporative Emissions System MalfunctionEGR System Malfuncion
The first thing that I would do is make sure no one has ran out of gas in the vehicle recently. This will cause this code almost every time. The next thing is to clean the MAF sensor. You can buy cleaner specifically for this procedure.
Then you should check for vacuum leaks, especially ones that are after the MAF like tears in the tube that goes from the airbox to the engine. If this is torn it will let air in that the computer cannot tell is coming into the engine.
The next most common cause is bad Oxygen sensors. If they are not working properly then they will not be reading the amount of oxygen in the exhaust gas.
These are the easier things to check. If none of this fixes it then it would be a good idea to take your car to a professional.
Mass Air Flow (MAF) Sensor
See Figure 1
Fig. Fig. 1: Terminals C and B supply voltage to the MAF sensor
WARNING Do not pierce the wires when testing this sensor; this can lead to wiring harness damage. Backprobe the connector to properly read the voltage of the HO2S.Single Wire Sensor
Heated Oxygen Sensor