Fuel System Too Lean (Bank 1 and 2)
This is a discussion of the meaning and common causes of the following OBDII codes:
P0171 Fuel System Too Lean (Bank 1)
P0174 Fuel System Too Lean (Bank 2)
This guide is for you if your check engine light is on and you’ve used an OBDII scanner tool to read the fault codes and they returned P0171 and P0174.
Common Symptoms In Your Car May Be:
- Check Engine or Service Engine Soon Light is on
- Rough Idle
- Lack of performance or power when accelerating or a misfiring engine
- In many cases, there may be NO noticeable signs of engine trouble
Why Have These Codes Been Logged?
Basically because your engine fuel mixture has too much air and not enough fuel in it (a lean mixture).
Your engine operates at an optimal fuel/air mixture that is controlled by the Engine Control Module. When the mixture is not an optimal ratio it are defined as:
Rich Mixture = Too much fuel, not enough air
Lean Mixture = Too much air, not enough fuel
The Engine Control Module measures the amount of oxygen in the exhaust system via the oxygen sensors and makes adjustments by adding more or less fuel to the mixture. When the adjustments become too large to maintain the proper mixture the fault codes P0171 and P0174 are logged. They are logged together since in V8 and V6 (and more rarely some 4 cylinder and straight 6 cylinder cars) the cylinders are split into 2 separate groups, or banks.
Be sure to search for other information that might be specific to your car make and model as pertains to these codes. There may be other very specific (or common) issues that other motorists have experienced with your car. Just Google the codes followed by your car make and model, ie: “p0171 and p0174 ford expedition“.
As one example, many report a common cause of the P0171 and P0174 lean mixture fault in the BMW X3 as the result of a faulty DISA valve.
Common Causes of P0171 and P0174
1) Vacuum Leaks
This is the most likely cause of too much air being taken into the system. The source of a vacuum leak could be one of many things, including (but not limited to) cracking, wear, or a hole in any of the following:
- Vacuum Hoses
- Air Intake Boots (see image example below)
- PCV Hoses
- EGR Valves
- DISA Valves
- A loose dipstick (or one with a broken seal)
If any of these parts are not forming a perfect seal (have a slit in them) then unmetered air will be entering the engine. Try to listen for a whistle or hiss under the hood with the engine running and visually inspect all hoses and connections.
2) Mass Airflow Sensor (MAF)
The MAF could be under reporting the amount of air passing through the intake. This could be caused by a dirty or fouled MAF or the MAF could be defective.
3) Engine Computer Software May Need To Be Updated
The Powertrain Control Module (PCM) software may need to be updated. As an engine wears the PCM Fuel Map may not properly compensate. This seems rather unlikely but worth checking into all the same.
4) Fuel System Obstructions
This could include:
- Clogged Fuel Filter
- Weak Fuel Pump
- Clogged Fuel Injectors
This is a case in which air flow is normal but not enough fuel is being delivered. Due to a possible loss of fuel pressure or flow the ECU cannot provide enough fuel to the attain the correct mixture.
5) Intake Manifold Gasket Leaking
If the engine is a V8 or V6 and only 1 side or bank is reporting a fault code it could be a leaking intake manifold gasket or leaking manifold. It is unlikely however that if you have logged both codes this would be the issue since both manifolds would have to be leaking.
What About the Oxygen Sensors?
This is a common misdiagnosis. While it is possible that this problem is a result of faulty O2 sensors it is highly unlikely. If you received both P0171 and P0174 banks detecting as lean that would mean that both O2 sensors are misreading the amount of oxygen in the exhaust. Not a likely scenario. Also, before these codes are possible the computer will run validation tests on the readings of the O2 sensors before looking at fuel trim adjustments. It would be then that a too lean mixture reading would be returned and logged as a fault P0171.
How To Read Your OBDII Computer Codes?
Read this tutorial on how to use one particular OBDII Bluetooth Code Reader:
How To Read OBD2 OBDII Codes
Get an inexpensive OBDII reader like the ones in the links provided above, plug it in and read away…