If the powertrain control module (PCM) determines that air-to-fuel is too lean, the PCM will produce a P0171 code. The P0171 code indicates that the PCM believes the engine is getting too much air compared to how much fuel is being delivered.
The PCM constantly receives signals from the oxygen sensors to modulate how much air and fuel it adds to the air-fuel mixture it delivers to the combustion chamber while awaiting a stable set of readings from the O2 sensors.
In this case, the engine’s fuel requirement is now outside of the factory preset fuel requirement range and would require less fuel (arguably less fuel is just approaching the theoretical minimum), thus the PCM produces the P0171 code.
In this instance, the PCM commanding less fuel is going to request more fuel to be delivered into the combustion chamber by the fuel injectors. This is referred to as fuel trimming, and here with the P0171 code, we would refer to it as positive fuel trim!
What Does the P0171 Code Mean?
The code P0171 means “Fuel Trim System Too Lean (Bank 1)”.
A P0171 is triggered when a significant increase in fuel is added from air being based on the readings from a O2 sensor. P0171 which in turn will trigger the check engine light.
P0171 could also indicate oxygen sensor is reporting to much oxygen due to significant lean/slightly lean fuel mixture.
Referring to the fuel lack as lean fuel means if we simplify everything down, there is running is too lean and the system is underperforming.
NOx gases are emitted into the atmosphere due for the combustion byproduct when using leaner fuel mix vehicles which has a greater environmental impact than normal richer mix vehicles. Leaner vehicles will misfire on a time to time basis as hydrocarbons from unburned fuel slowly burn and reach the catalytic converter.
Possible side effects with running a P0171 vehicle is accelerated fuel use, potential damage to exhaust system, and failure to meet emission standards.
For information regarding rich and lean OBD codes, please reference our discussion about the air-fuel mixture. You may also refer to our discussion about the ideal mixture and OBD codes if you are interested in understanding how the air-fuel mixture can trigger OBD codes would be technically stored on a vehicle.
At this point what you should do is move ahead to the next section to obtain information quicker to assist in understanding the potential issues behind the P0171 code.
When this code is set, oxygen sensors detect low levels of oxygen that remain unchanged in the exhaust (lean), therefore the module responds by trying to add fuel to the system more than it would normally to the balance, to the amount of air/fuel mix.
- Rich Mixture = Too much fuel, not enough air
- Lean Mixture = too much air, not enough fuel
P0171 Code Symptoms
Sometimes, you may notice your check engine light comes on, and the car stays and drive the same. After it has been scanned for trouble codes, it may show additional symptoms, and a reading or scan shows the code P0171 with other codes.
“What that tells me is that there is too much air in the engine.” If your engine is idling when it is running for a few minutes and you are not moving, it will probably give you this quality, besides a cough sound.
As you drive, you may feel that your car is fighting to gain power when trying to accelerate, and when you are driving uphill or during this time, you might feel some engine misfire as well.
If you do not feel any difference while you are driving your car with a P0171 code, and if the performance does not change at all, you may just have a bad sensor, which we all know these parts can malfunction.
However, even if you do not see any other symptoms, you have to make sure everything close to the sensor is working as it should too. It is important to make sure the sensor is not damaged, as it would be frustrating to change expensive parts that did not need replacing.
Common causes of P0171 Code
When you check a vehicle with a stored P0171 code, the first thing to check is the vacuum system. Vehicle vacuum lines can develop small tears or cracks (or develop holes), which will also change the normal distribution of air flow.
There are several leak sources that could be the cause of the problem code P0171: engine intake, intake manifold gasket, PVC hoses, and vacuum hoses. If the vacuum lines have not never had a proper visual inspection and have no visible tears or cracks, then you can reasonably assume that your issue is the fuel system.
Not enough gasoline is getting to the pump and the supply gets terminated within either a weak fuel pump, or failing/full banishment of the fuel pump.
This assumes that the complete vacuum system has been properly tested for leaks, that gas is getting to the engine, and you still have code P0171 in the electrical system.
The real problem can be found by determining if and identifying which sensor is malfunctioning, or failing; either a dirty Oxygen sensor, mass airflow sensor, or air-fuel ratio sensor. They may be having trouble providing informative readings; a sensor’s readings could also be affected by an exhaust leak.
Is Code P0171 Serious?
Check engine light: something is wrong, how serious or not the given codes may be should not be overlooked. And for which reasons. Same goes for P0171. Of course, if any travel is needed, with consideration to the reason for the intended travel, it makes more and more sense to limit the range of operation of the vehicle.
Operating a car with severely insufficient fuel supply or air and fuel supply is an extremely bad situation, and in terms of vehicle performance, inability to successfully accelerate with the following traffic, or travel up any steeper incline is also a consideration, plus overheating could mean major damage in a lesser amount of time depending on the load placed on the engine.
Turbocharged vehicles that someone has been fooling with or bolstering in power will be more prone to display P0171 code. Power/air has a correlation, however, the need for good fuel supply still needs to be obtained to achieve optimal air to fuel ratio in not being detrimental to performance.
Worst case scenario for a vehicle, like a vehicle that has problems with low fuel supply to the engine, like in the case of a vehicle that has the displayed code P0171, experiencing poor fuel economy, is higher costs in trying to operate it. The car is likely wasting money as the fuel cost will surely surpass what it is worth to provide efficient operation for the vehicle.
How to Diagnose Code P0171?
Make sure to complete a thorough OBD-II scan of your automobile and not rush to conclusions about the reason for the underlying issue. A Code P0171 needs to be verified by you or a mechanic. If this is the case for your problem and you do not have any other issues, this is likely your diagnosis while you see a check engine light.
It is easy to think replacing a few sensors will get your problem fixed fast, but this will show to you with an OBD-II scan that your automobile issues occur related directly around your Bank 1 area. Having someone who has a history of solving Code P0171 issues look at your problem is useful.
To fix it, you will need to clean the workspace thoroughly to access the area that is now restricted.
You will identify Code P0171, and follow these steps in order until you can find the source of the error message, and you should be able to find something that has value.
- Test the vacuum gauge for leaks with the engine off.
- Use a fuel pressure measuring device to test fuel sensors and mass flow air sensors for value.
- Test oxygen sensors all through the fuel system per the manufacturer’s specified procedures.
- If everything else doesn’t work, the problem is most likely the power control module.
How Do I Fix the P0171 Code?
The error for the Bank 1 system running too lean can be addressed in a variety of ways. Some of the fixes may be enough to turn off the check engine light:
- Replacing a fuel pump or filter
- Changing a fuel pressure regulator
- Replacing a control module
- Changing one or more fuel injectors
- Changing one or more oxygen sensors
- Changing the mass air flow sensor
- Fixing a vacuum leak
Completing maintenance, which could include oil changes, tune-ups, and general routine servicing, is one way to mitigate the P0171 code from reappearing.
Cost to Fix Code P0171
These prices will vary depending on location, any additional fees for repairs, and the type of vehicle being serviced.
- Vacuum/PCV Hose. The average cost for parts and labour combined for a vacuum hose or PCV hose replacement is between $90 to $190. Typically the cost of the parts will be in the range of $10 to $50. The labor will vary from $80 to $140 depending on the shop that installs it.
- Intake Manifold Gasket. Expect a price for replacing your intake manifold gasket to run between $100 and $500. The gasket and bolts will typically be $20 to $100, and the labor costs can range from $80 to $400 depending on your vehicle.
- Fuel Pump. Replacing your fuel pump is a more expensive service and ranges from $300 to $800. Depending on your vehicle the pump could run $100 to $400. Labour charges would be anywhere around $200 to $400.
- Fuel Injector. The price for replacing a fuel injector may run $130 to $800, depending on your vehicle details. The fuel injector itself could cost $50 to $300. Labour costs could run $80 to $500.
- Fuel Filter. Replacing a fuel filter is less expensive service and can range from $90 to $190. The fuel filter could run $10 to $50. The labour is expected to range from $80 to $140 to accomplish a replacement.
- Oxygen Sensor. If you have an auto shop replace an oxygen sensor, the costs are generally between $100 and $400. However, the final costs depend on your local area and the auto shop itself. The average cost of an oxygen sensor is perhaps $20 to $200, and the fees to replace it are probably around $80 to $200.
- MAF Sensor. The replacement cost for a Mass Air Flow (MAF) sensor will range from $200 to $500. The auto service provider is going to charge you about $100 to $300 for the actual MAF sensor, and $100 to $200 for labour.
- Air/Fuel Ratio Sensor. The replacement expected price for the air/fuel ratio sensor will probably be around $130 to $500. If the sensor part itself runs $50 to $300, you will also be charged $80 to $200 directly for labour.
Why Should a P0171 Code and Running “Too Lean” Matter?
“Lean” running cars and light-duty trucks are classified as “gross polluters.” A startling number of individuals suffering from respiratory ailments caused by “lean running” vehicles cause many to suffer from asthma.
Both lean and rich vehicles have their issues, but the lean-running vehicle has misfires to boot.
Such misfires introduce uncontrolled hydrocarbons (HCs) into the environment, which causes injury, such as the destruction of the car’s catalytic converter. The end-result is deadly fumes that pollute the air while causing humans to burn their eyes in the process.
They say it’s not all bad, if we look at the rich running engine, which, while causing enough “odorless” carbon monoxide (CO) or, as some might say, the “rotten egg” sulfur dioxide released from the catalytic converter, a P0171 code does not come from a malfunctioning oxygen sensor.
By definition, before identifying a P0171 code, there is always a series of checks and tests performed to ensure there is adequate oxygen sensor readings.
As previously mentioned in the readiness tests for the oxygen sensors, considering it not setting any codes, it stands that the computer is more apt to tend to the Fuel Trim adjustments. The first time the computer identifies that the a: F ratio is too lean, the code P0171 would be set.
What If There Is No Vacuum Leak, and the P0171 Code is Set?
Another common cause for a code P0171 is an “underreporting” Mass Air Flow Sensor. In essence, that’s saying that the Air Flow Sensor is indicating less air is in the engine than there is.
The computer gets more fuel requests from the oxygen sensors and gets confused between the mass air flow sensor saying there is too little air and oxygen sensor telling the computer the mixture is still too lean!
It tries to compensate but, since it’s impossible to arrive at a resolution, it sets the code. Let me clarify; the oxygen sensors are accurate—the fuel mixture is too lean.
The Mass Air Flow Meter is reporting all wrong about how much air is entering the engine.