Diagnosing and fixing the Diagnostic Trouble Code (DTC) P0171: System Too Lean requires verifying the vehicle’s ability to accurately measure and meter the air/fuel mixture that enters the combustion chamber. If all sensors are performing to manufacturer specifications, tracking down the source of an air or fuel leak is the next logical step. Engines that cannot maintain at or near the optimal 14.7 parts air to 1 part fuel run the risk of increased friction and premature failure of the internal components.
The guide below explores the common symptoms and causes of the P0171 DTC code to help car owners better understand the steps involved in diagnosing and repairing the root of the problem. However, it may not have information specific to the make and model that you’re working on. For comprehensive, vehicle-specific repair information, DIY mechanics and automotive repair professionals can leverage Identifix Direct-Hit Pro’s database of confirmed fixes and virtual technician hotline to find the appropriate solution faster. The addition of the new Direct-Hit Mobile experience ensures instant access to the critical information you need from the palm of your hand.
DTC P0171 refers to an imbalance in the amount of air and fuel entering the engine as the vehicle operates. Specifically, the “System Too Lean” description indicates there’s more air or less fuel in the system than should typically be there. The engine code P0171 can illuminate the check engine light (CEL) on any vehicle, making it a generic code that spans manufacturers and individual models.
In a nutshell, the amount of air flowing into the system — as measured by the mass airflow (MAF) sensor — dictates how much fuel gets injected into the combustion chamber. The oxygen (O2) sensor then evaluates the amount of air and fuel burned to ensure proper calculations. The powertrain control module (PCM) or engine control module (ECM) trims the fuel as necessary to maintain the desired air/fuel ratio, but when the trim exceeds the system’s parameters — in the short or long term — the system throws the P0171 code.
Similar to any code that illuminates the CEL, P0171 should be diagnosed and resolved sooner rather than later. The P0171 code points to issues with the critical processes that allow your vehicle to function. Prolonged use can only further damage the corresponding components, including your car’s catalytic converter, spark plugs, pistons, etc.
The following symptoms that correspond with the P0171 engine code provide a somewhat imprecise measure of the root cause of the issue. However, they can be useful in removing potential causes by process of elimination. Common symptoms of DTC P0171 include:
Engine code P0171 is often accompanied by P0170, which is also related to fuel trim and an imbalance in the air/fuel ratio. Codes P0300 and P0308 — which refer to misfiring cylinders — can also be triggered. A lean engine is more prone to misfire.
Most causes of the P0171 code are related to fuel or air. Fuel-related issues stem from a lack of fuel or pressure. Unmetered air and vacuum leaks contribute to too much air in the system, but P0171 can also be caused by a larger issue that affects engine operation.
As a fuel pump ages, the motor’s components can wear, making it less efficient. If the pump is unable to feed the system with enough fuel, it can trigger the P0171 code and the CEL. A quick test of the fuel pump’s power helps rule out this common cause.
If the hangup in fuel delivery doesn’t happen at the fuel pump, the fuel injectors may be the culprit. Individual fuel injectors can fail to spray the proper amount of gasoline into the engine. The number of fuel injectors technicians must inspect depends on the engine’s design.
Similar to a defective fuel injector, a clogged fuel filter can impede the flow of fuel. Since the contaminants act as restrictions in preventing a streamlined flow, the engine suffers from modulating fuel amounts, which the system can find challenging to account for. A rough idle or misfire can stem from a clogged fuel filter that initiates a P0171 engine code.
The fuel pressure regulator is responsible for properly managing fuel pressure within the fuel system. When this component fails, the system no longer functions as it should. The regulator cannot adjust fuel for acceleration, turn excess fuel to the gas tank, or regulate fuel pressure for optimal output from the injectors.
A vacuum leak due to a compromised positive crankcase ventilation (PCV) valve, intake manifold gasket or hose, or vacuum line introduces unaccounted-for air into the system. Sealing this leak limits the air and allows the ECM to accurately balance the air/fuel mixture. Vacuum leaks can be as small as a tear or as large as a gap between components.
As the name suggests, this sensor measures the air/fuel ratio in the combustion chamber. The AF sensor works with the O2 sensor to provide feedback to the ECM. When the AF sensor detects an imbalance in the air/fuel ratio, it triggers a P0171 warning.
Over time, an MAF sensor can easily become clogged with contaminants suspended in the air. Like your engine and cabin air filters, MAF sensors must be clean to perform correctly.
An exhaust leak before the final O2 sensor can trigger a P0171 code. Essentially, the O2 sensor returns a different reading than what the MAF sensor indicates. The ECM looks at the feedback loop between the two to determine the fuel trim, but an exhaust leak can continue to cause the O2 sensor to read a fault in the system.
As one of the primary sensors that measure air pressure within the intake manifold, the MAP sensor can also cause the ECM to modulate fuel delivery to account for additional air. However, if the MAP sensor returns inaccurate readings, the ECM may inadvertently be trimming fuel unnecessarily. A failing MAP sensor can also cause a rough idle or stalled engine.
One of the last components to measure the air/fuel mixture, the O2 sensor can sometimes return faulty readings due to age or wear. It’s critical to compare the O2 sensor’s data with that of the MAF to determine the weak link.
Beyond the basic tools required for diagnosis, an OBD-II scanner and multimeter make it easier to obtain and verify readings. The multimeter allows you to check the function of the many sensors involved with this DTC. In addition, diagnosis and repair may require a fuel pressure sensor gauge, smoke machine, and MAF cleaning agent.
The ultimate goal of diagnosing a P0171 engine code is to determine what component(s) caused the engine to run lean in the first place if indeed it was running lean. Through a process of visual inspection and testing protocols, you can narrow down the potential causes to identify the appropriate fix. Begin by plugging in the OBD-II scanner, verifying the P0171 code, and noting any accompanying DTCs.
Examine the air intake system to ensure there are no cracks or inadequately sealed components. Check vacuum hose fittings, especially those around the PCV valve and intake manifold.
Use procedures recommended by the vehicle’s manufacturer to test the MAF sensor. If cleaning is necessary, be sure to use only an approved cleaner to avoid damaging the MAF. As you remove the MAF, inspect the intake boot for cracks or any signs of wear.
Using a smoke machine, test the exhaust system for leaks to determine the source of unmetered air in the system. You may perform this test several times to ensure a properly sealed exhaust, especially if you tightened a few connections.
Verify the fuel pressure ratings are within the manufacturer’s recommendations. Remove and inspect the fuel filter, cleaning it if necessary. Ensure the fuel injectors, fuel pump, and fuel pressure regulator are all performing within specifications.
Ensure the O2 sensor is in proper working order. Compare the air/fuel readings measured by the system against those calculated by the O2 sensor. If there is a discrepancy, the O2 sensor may be to blame.
After running the vehicle for a short time, turn off the engine and remove the spark plugs. Inspect the tip for a white haze or crust and replace spark plugs if present. If the problem persists, further diagnosis is needed to determine any additional causes.
If you haven’t run into the cause of the P0171 code by this point, chances are you’re dealing with a faulty PCM or ECM. Before proceeding with this type of repair, ensure all other causes have been ruled out. Repairing or replacing this component requires a separate set of diagnostic procedures.
It’s important to remember that these general steps for diagnosing and fixing a P0171 code may not address every potential cause, especially as they vary by vehicle make and model. Because different vehicles can have unique fuel trim and sensor configurations, relying on general troubleshooting may only take you so far. For accurate, model-specific guidance, consider using Identifix Direct-Hit Pro.
Direct-Hit Pro offers access to an extensive library of confirmed fixes, OE service manuals, and an interactive technician hotline, making it easier to pinpoint exact causes and recommended repairs for specific vehicles. By using Direct-Hit, you can ensure that your repair approach is both efficient and effective, tailored to the exact needs of each vehicle. With Direct-Hit Mobile, mechanics can even troubleshoot in real-time right from the vehicle, ensuring the fastest path to resolving the P0171 code accurately.
Contact us today to learn more about how Direct-Hit Pro can streamline your workflow and give you model-specific insights to resolve code issues like P0171 effectively.
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