OBD II Fault Code:

P0171 Fuel System Too Lean (Bank 1)
P0174 Fuel System Too Lean (Bank 2)

Symptoms:
Check Engine Light will be illuminated
In some cases no adverse conditions may be noticed by the driver
In other cases there may be performance problems, such as a lack of power on acceleration and even some ‘coughing’ or misfiring. The vehicle may have trouble idling, especially when
cold or when sitting at a stoplight

Common Problems:

Vacuum Leaks (Intake Manifold Gaskets, vacuum hoses, PCV hoses etc.)

Mass Airflow Sensor (MAF)
Plugged Fuel Filter or weak Fuel Pump
Plugged or dirty Fuel Injectors
Common Misdiagnosis:

Oxygen Sensors
Polluting Gases Expelled:

NOX – Oxides of Nitrogen -> toxic gases that adversely effect breathing and are a major contributor to smog
HC’S –Hydrocarbons -> unburned droplets of raw fuel

The Basics:

Combustion engines operate by burning an air/fuel mixture of about 14.7/1 (14.7 parts air/1 part fuel). When the air ratio goes above 14.7 parts, this is called a “lean” mixture, and if the air drops below 14.7 parts it is called a “rich” mixture.

Lean Mixture = Too much air, not enough fuel
Rich Mixture = Too much fuel, not enough air

To keep the engine running properly within this 14.7/1 mixture, the Engine Control Module measures the oxygen content in the exhaust with “Oxygen Sensors” and makes adjustments to the mixture by injecting more or less fuel with the fuel injectors.

The control module operates within specific parameters, and under normal conditions it will make minor adjustments to the air/fuel mixture.
When these adjustments become too large a fault code is set. In this situation (P0171, P0174) the oxygen sensors are detecting too much oxygen in the exhaust and the control module is adding more fuel than normal to sustain the proper air/fuel mixture. (Lean Condition)

P0171-P0174 Diagnostic Theory for Shops and Technicians:

When a vehicle has the fault code P0171 and or P0174, it means that the automatic mixture adjustment that is made by the computer is at its maximum.
Code P0171 is described as "Fuel System Too Lean Bank 1" where as Code P0174 is "Fuel System Too Lean Bank 2". Code P0174 mainly applies to V6 or V8 engines because 4 cylinder engines generally have 1 bank. There are a few exceptions.
Some of the high performance 4 cylinder and straight 6 cylinders engines split the cylinders into groups of 2 or 3 and call them separate banks, such as Subaru, BMW or Lexus.

When the code says that the Fuel System is "Too Lean", it means that the computer has been adjusting the fuel mixture richer and adding more and more fuel.
This is called Long Term Fuel Trim. Ideally, the Long Term Fuel Trim should be close to 1-2%. When a code P0171 is flagged, it means that the Fuel Trim is any where from 15% to as high as 35% compensated.
When this happens, the computer knows that there is an irrational condition in the control of the Fuel System. The first step in the diagnosis of a code P0171 and/or P0174 is to look at a minimum of 3 ranges of the Long Term Fuel Trim numbers on a scanner.

Check the idle reading, 3000 rpm unloaded and 3000 rpm with at least 50% load. Then check the freeze frame information for the code to see which range(s) failed and what the operating conditions were. Before we get into the main causes of P0171, lets explore why this code matters.

Why should we care if the Fuel System is too lean? And why this should be addressed!

Lean running cars and light trucks are highly polluting vehicles. Most of the NOx pollution is caused by the vehicles that are running too lean. NOx is very poisonous, brings on Asthma and causes the sky to turn that putrid yellow color. A lean running car also begins to misfire which puts raw fuel or HC's into the atmosphere which are also very poisonous and color the sky.
When you are behind a car or truck and it smells very bad and makes your eyes burn, it is misfiring and running too lean. A rich running engine has no smell ( CO is odorless ) and the only thing you may smell is a little of that rotten egg smell from the sulfur being burned by the Catalytic Converter in the extremely rich cases like when you are behind a big SUV at full throttle going up a long steep hill.

P0171 is NOT an Oxygen Sensor Problem. In order for the code P0171 to set, the Oxygen Sensors had to have passed their readiness tests and not set any pending or persistent codes. This means that the computer has run a series of tests to validate the data that is received from the Oxygen Sensors. Then it looks at the Fuel Trim adjustment.

So, what are some common causes for P0171?

A vacuum leak is very common.
It could be a torn PCV hose, a torn Intake Air Boot, or even a broken seal on the dipstick because, the dipstick is a part of the PCV system and if it does not seal, too much unmetered air ( air the computer can’t measure or ‘meter’ ) will enter the engine.

Don’t rule out a sticking/leaking EGR Valve or leaking EGR or Intake Manifold Gasket. If it is a V6 or V8 engine and the code is only on 1 side (Bank), it could very well be a defective Intake Manifold Gasket or cracked/leaking manifold.

What about when there is a code P0171 and there is no vacuum leak found or both code P0171 and P0174 set?

A Mass Air Flow Sensor that is ‘under reporting’ to the computer can be a common cause of a code P0171 or on a V engine codes P0171 and P0174.

Basically what this means is that the Air Flow Sensor is telling the computer that much less air is entering the engine than the actual amount. This causes confusion between the Oxygen Sensors that tell the computer that more and more fuel is needed.
The computer gets confused, because it sees the measurement from the Air Flow Meter or Sensor and then it hears the Oxygen Sensor complain that the mixture is still too lean. The computer keeps trying to compensate until it gets ‘confused’ and sets the code.

It is important to re-state that the Oxygen Sensors are telling the TRUTH! The Fuel Mixture is too lean. In this case the Air Flow Meter or Sensor is not doing its job of reporting the real amount of air entering the engine.

There is a very effective ‘truth test’ for any Mass Air Flow Sensor. Start the engine and let it idle and then check the Barometric Pressure reading on the scan tool data.

If the reading is say 26.5 and you are close to sea level, you know that you have a defective Air Flow Meter because it is telling you that you are at about 3500 ft above sea level.
You can find conversion tables for these reading below.

The Barometric Pressure Sensor is part of the Mass Air Flow Sensor and will cause the Mass Air Flow Sensor to send incorrect data to the engine control module. Sometimes the Air Flow Sensor gets dirty and the sensing wire gets covered with dust or oil residue. This can also cause a P0171.

Cleaning the sensor often works for a while but eventually the MAF sensor needs to be replaced. Always make sure the Air Filter and Air Filter enclosure is dirt and dust free.

Clean and replace as needed or the new MAF will fail in a short amount of time.

Some of the other causes of P0171, but less common are a plugged fuel filter or poorly functioning Fuel Pump. The computer hears, truthfully, from the Oxygen Sensor that the Fuel Mixture is too lean and the computer keeps increasing the amount of fuel being delivered into the combustion chambers but in this case, the Fuel System can’t increase the amount of fuel.
Be sure to check and verify that fuel pressure and delivery are at spec if the above inspections do not reveal any problems.
If fuel pressure and volume are good scope the injectors and if needed perform injector drop and or flow tests to see if they are capable of delivering enough fuel. Dirty/contaminated gas can definitely plug injectors and trigger these lean codes