So, your vehicle’s check engine light has come on—you then plug in your On-Board Diagnostic II (OBD-II) scanner and it sets the P0171 code. If you’ve never encountered this code before, you may be at a loss as to what could have possibly caused it.
This comprehensive guide will explain everything you need to know about this diagnostic trouble code, including symptoms, causes, how you can fix the issue, and more.
What Does Code P0171 Mean?
Code P0171 or Fuel System Too Lean (Bank 1) is a diagnostic trouble code (DTC) that is set when the engine control module (ECM) detects an imbalance in the air-to-fuel ratio in Bank 1 of your engine.
The P0171 code refers to a lean condition in Bank 1, which is the side of the engine where cylinder 1 is located. In simpler terms, “lean” means there’s too much air and not enough fuel in one or more cylinders on Bank 1.
Note: Many engines only have one bank, but larger models (V6, V8, V10, W8, etc.) have a second bank on the opposite side where you’ll find cylinder 2.
When a P0171 code is detected, the powertrain control module (PCM) attempts to compensate by using its fuel trim algorithms to add more fuel to balance the air-to-fuel mixture. If you use a scan tool with a datastream feature, you’ll typically observe positive long fuel trim (LFT) values in double digits, showing the PCM has made a significant adjustment. Meanwhile, short fuel trim (SFT) values might reset to zero, as the LFT adjustment compensates to maintain balance.
While the PCM’s algorithms can keep the engine running temporarily, this issue should be addressed to prevent long-term damage and maintain optimal performance.
What Causes Code P0171?
It’s essential to understand the root cause of the P0171 code so you can address the issue that caused it. Here are the top culprits behind this trouble code:
- Vacuum leaks
- Clogged MAF sensor
- Insufficient fuel delivery
- Exhaust manifold leaks
Vacuum Leaks
This is the most common cause of the P0171 code, also known as an “intake air leak.” However, on mass airflow (MAF) systems, this will be referred to as “unmetered air,” which means air is entering the intake steam that isn’t being factored into the PCM’s calculations. This drives the O2 sensor to signal “lean,” and the PCM responds by adding enough fuel to bring the O2 sensor’s signal back to where it needs to be.
When the SFT remains positive for a long time, LFT numbers are forced to correct the mix to bring the SFT back to zero. When the LFT remains in the positive double-digit range for too long, the P0171 code is set.
Vacuum leaks allow unmetered air to fill up the combustion system, which disrupts the balance between the air and the fuel. These can come from cracked hoses, intake manifold gaskets, or faulty PCV valves. The system is so sensitive that even a loose oil filler cap (or a missing oil filler cap gasket) will cause the PCV system to draw unmetered air in through the crankcase and cause this issue. Using the wrong PCV valve, such as one with too much flow, can cause this as well.
Clogged MAF Sensor
The MAF sensor measures incoming air and provides data to the ECM. When this sensor is dirty or malfunctioning, it can send incorrect airflow readings that may result in an improper mixture.
Insufficient Fuel Delivery
Another issue can be the decreasing amount of fuel entering the combustion chamber, caused by a weak fuel pump, blocked fuel filter, or rust-clogged injectors.
Exhaust Manifold Leaks
When exhaust manifold leaks are upstream of the oxygen sensor, that oxygen will cause the sensor to report a lean condition when there is none, leading to inaccurate readings and allowing the ECM to believe there is a lean air-fuel mixture.
What are the Symptoms of Code P0171?
You can detect the presence of a P0171 code through more than just your scanners. Here are some signs you should look out for to help you fix this issue before it worsens.
- Check engine light is on
- Rough idle
- Reduced fuel efficiency
- Engine misfire
Check Engine Light is On
Everything starts with your check engine light, which is the most obvious sign. When the air-fuel mixture becomes too lean and the engine can no longer compensate by adding fuel, the PCM turns the check engine light on and sets the trouble code.
Rough Idle
An engine running lean makes it hard to maintain a smooth idle. But because of the fuel trim algorithms described earlier, the engine won’t run rough except in extreme cases.
Reduced Fuel Efficiency
This lean condition of your engine causes the ECM to compensate by injecting more fuel to restore the air-to-fuel ratio. Sometimes, this may cause your fuel efficiency to take a hit and your overall mileage to perform lower than expected. While this barely happens, there won’t be a noticeable difference when it does.
Engine Misfires
Engine misfires may cause a P0171 code, but it won’t be because of a lean condition. Remember, the O2 sensors can’t smell fuel; they’ll only report the excess oxygen detected in the exhaust stream from a misfiring cylinder.
How to Diagnose Code P0171
Before you shift straight into repair mode, it’s crucial to be systematic in identifying the root cause of the P0171 code.
Look for Vacuum Leaks
Inspect your vacuum hoses, intake manifold gaskets, and PCV valves for any cracks, damage, or disconnections. There might be a hissing sound coming from the engine bay if there’s a vacuum leak. You can also try a smoke test (using a smoke machine) to locate hidden leaks.
Measure Fuel Pressure
Check the fuel pump, filter, and injectors with a fuel pressure gauge to ensure proper fuel delivery. With the gauge connected, snap the throttle and see if the fuel pressure drops. If it does, replace the filter, retest, and, if necessary, replace the pump.
Inspect the MAF Sensor
Remove the MAF sensor from the air intake system and visually inspect it for dirt or physical damage.
Check the PCM
Once all diagnostic tests are finished and no apparent issues come up, it might be a problem with the PCM. While this seldom happens, a problem with the PCM is a problem with the entire vehicle. The PCM acts as the “brain” of the engine system, and one issue left unchecked, no matter how minor, can lead to a range of other issues, ultimately resulting in the P0171 code.
How to Fix Code P0171
Here are the most effective solutions to get your vehicle back on track if you have a P0171 code stored.
Resolve Vacuum Leaks
Seal any vacuum leaks by repairing or replacing cracked hoses, worn gaskets, or faulty PCV valves. Once these leaks have been fixed, your vehicle’s air-to-fuel ratio will soon go back to normal.
Pro Tip: When repairing a vacuum leak, remove the battery cables so the PCM will forget its learned fuel trim. That way, both the LFT and SFT will start at zero. When the system enters a closed loop, watch the SFT. If it hovers near zero and stays there, you’re done. If it starts moving into positive territory again, it means you have not repaired all your unmetered air leaks.
Replace (or Clean) Affected Components
For a dirty MAF sensor, use a specialized cleaner to wipe and scrub away dirt or buildup. Or, if it’s damaged, just replace it with a new one.
The same goes for the O2 sensors—if they’re showing signs of malfunction, you’ll need to change them, too. Typically, when conventional O2 sensors begin to fail, they’ll read lean, but first, they’ll become very sluggish, which sets a different code.
Finally, replacement is your best option with a weak fuel pump, a faulty fuel pressure regulator, or defective injectors.
Recalibrate the ECM Software
Sensor readings tend to become less accurate over time as the ECM software becomes more outdated. If all else fails, consult a professional to help you update the software.
Other Codes Related to P0171
Here are some of the codes that could appear with a P0171.
P0174
The P0174 code indicates a lean condition in Bank 2, often caused by similar issues with the air-fuel mixture.
P0101
This is one of the most common OBD-II codes, which signifies a problem with the MAF sensor, such that it may be out of its intended range. These issues may also cause the P0171 code to appear.
P0300
A P0300 code refers to random cylinder misfires within your engine, which can occur due to the lean air-fuel mixture that made the P0171 code pop up in the first place.
If two or more of these codes appear at the same time, it’s best to look into larger internal issues within the system like vacuum leaks or fuel delivery problems.
FAQ
Will a P0171 code clear itself?
The P0171 code might temporarily disappear if the issue is intermittent, but the code can come back if the problem persists. It’s important to address the underlying cause and resolve the issue as soon as possible.
What causes P0171 and P0174 at the same time?
When both the P0171 and P0174 codes appear, it means that both of your engine banks have an imbalance in the air-to-fuel mixture. This points to a larger issue within your vehicle, caused by things like a major vacuum leak, faulty mass airflow sensor, or low fuel pressure.
Can I drive with a P0171 code?
Yes, you can, but that doesn’t mean you should. While you will physically be able to operate your vehicle even with the P0171 flashing, it’s strongly not advised to do so. A lean system leads to high engine temperatures, which can only cause other misfires and damage your vehicle over time.