If you own a car or work on cars, you may have encountered a P0171 fault code at some point.
This code indicates that the fuel system is running too lean, meaning that there is too much air relative to fuel in the engine. In other words, the engine is not getting enough fuel to run efficiently or effectively.
If left unaddressed, P0171 can cause various problems, including reduced performance, increased emissions, and even engine damage. Therefore, it’s essential to understand what P0171 is, what symptoms it causes, what causes it, how to diagnose it, and how to fix it.
I’m John Cunningham, a qualified mechanic, you are in the right place, and very shortly, you’ll have a good understanding of the cause, the diagnosis process, and the fix.
In this post, we will explore the following:
What is P0171?
As mentioned, P0171 is a diagnostic trouble code that indicates a fuel system running too lean. It is a standardized code that can be read by an OBD-II (On-Board Diagnostics II) scanner, which is a tool that can communicate with the engine control module (ECM) or powertrain control module (PCM) of a car to retrieve diagnostic information.
P0171 is typically associated with the following conditions:
- Bank 1 (the side of the engine where the #1 cylinder is located) runs too lean.
- The ECM/PCM has detected a long-term fuel trim that exceeds a threshold, meaning that the engine has been trying to compensate for a lean condition but has not succeeded.
P0171 is a generic code, which means that it applies to all cars that use the OBD-II protocol, regardless of the make, model, or year. However, some car manufacturers may use different codes or definitions for similar problems. Hence, it’s important to consult the car’s service manual or technical resources to confirm the specific meaning of P0171 for a given car.
P0171 can cause various symptoms, depending on the severity and duration of the lean condition and other factors such as driving conditions, temperature, altitude, and engine load. Here are some common symptoms of P0171:
- Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL) is on: This is the most obvious symptom of P0171, as the ECM/PCM will illuminate the CEL/MIL to signal that a fault code has been detected. The CEL/MIL may flash or remain steady, depending on the severity of the problem and the car’s programming. The CEL/MIL may also be accompanied by other warning lights or messages, such as Reduced Power Mode, Emissions System Problem, or Service Engine Soon.
- Rough idle or stalling: P0171 can cause the engine to run rough at idle or stall when stopping, as there is insufficient fuel to maintain steady combustion. The engine may shake, vibrate, or sound like it’s misfiring. The idle speed may fluctuate or drop below normal, causing the car to shake or stall.
- Hesitation or sluggishness: P0171 can also cause the engine to hesitate or feel sluggish when accelerating, as there is not enough power to overcome the resistance of the load. The car may feel like it’s dragging or bogging down, especially when going uphill or merging onto a highway. A jerking or surging sensation may also accompany the acceleration.
- Poor fuel economy: P0171 can reduce the car’s fuel efficiency, as the engine has to work harder to compensate for the lean condition. The car may consume more fuel than usual, or the mileage per gallon (MPG) may drop significantly. This can increase fuel costs over time and contribute to environmental pollution.
- Exhaust odor or smoke: P0171 can produce an unusual odor or smoke from the car’s exhaust, as lean combustion can produce excess heat, unburned fuel, and harmful gases. The odor may be described as sweet, pungent, or acrid, depending on the cause of the lean condition. The smoke may be visible as a white or gray cloud, indicating incomplete combustion or oil consumption.
- Engine damage or failure: P0171 can lead to serious engine damage or failure if left unaddressed for a long time, as the lean combustion can cause the engine to run hotter than normal, resulting in detonation, pre-ignition, or melting of critical components such as valves, pistons, or catalytic converters. This can result in costly repairs or even total engine replacement, depending on the extent of the damage.
- Other codes or symptoms: P0171 can be accompanied by other codes or symptoms, depending on the underlying cause of the lean condition. For example, P0174 may indicate a lean condition on Bank 2, meaning that both sides of the engine are affected. Other codes may indicate specific components or sensors that are malfunctioning, such as the oxygen sensor (O2S), mass air flow sensor (MAF), manifold absolute pressure sensor (MAP), or fuel injector.
If you experience any of these symptoms, especially the CEL/MIL, it’s recommended to have the car diagnosed and repaired as soon as possible to prevent further damage or safety hazards.
What causes P0171?
P0171 can be caused by a wide range of factors, some of which are more common than others. The following are some of the top causes of P0171:
Vacuum leaks: A vacuum leak occurs when air enters the engine from a source other than the intake manifold, such as a cracked hose, gasket, or valve. This can result in a lean condition, as the excess air dilutes the fuel mixture. Vacuum leaks can be hard to locate, as they can occur in various parts of the engine, including the PCV system, brake booster, EVAP system, or intake manifold gasket.
Faulty sensors: A faulty sensor can send incorrect signals to the ECM/PCM, causing the fuel system to operate improperly. The most common sensors that can cause P0171 are the O2S, MAF, MAP, and fuel injectors. The O2S measures the oxygen content in the exhaust and sends feedback to the ECM/PCM to adjust the fuel trim. If the O2S is faulty or contaminated, it may provide false readings that result in a lean condition.
The MAF measures the amount of air entering the engine and sends feedback to the ECM/PCM to calculate the fuel injection. If the MAF is dirty or damaged, it may underestimate the airflow and cause a lean condition. The MAP measures the pressure in the intake manifold and sends feedback to the ECM/PCM to adjust the fuel injection. If the MAP is faulty or disconnected, it may provide false readings that result in a lean condition.
The fuel injector sprays the fuel into the intake manifold precisely to match the airflow and throttle position. If the fuel injector is clogged, leaking, or stuck open, it may deliver too little fuel and cause a lean condition.
Low fuel pressure: Low fuel pressure can result in a lean condition, as the fuel cannot reach the injectors sufficiently. A weak fuel pump, clogged fuel filter, or faulty fuel pressure regulator can cause low fuel pressure. A fuel pressure test can confirm if the fuel pressure is within the manufacturer’s specifications.
Restricted air intake: A restricted air intake can result in a lean condition, as the engine cannot receive enough air to mix with the fuel. A restricted air intake can be caused by a dirty air filter, clogged air ducts, or a faulty air intake system. The air filter should be replaced regularly according to the manufacturer’s recommendations, and the air intake system should be inspected for any signs of damage or wear.
Exhaust leaks: An exhaust leak can result in a lean condition, as the excess air can enter the engine through the leak and dilute the fuel mixture. Exhaust leaks can occur in various parts of the exhaust system, including the exhaust manifold, exhaust pipes, muffler, or catalytic converter. Exhaust leaks can also produce loud noises, unpleasant odors, or visible smoke from the exhaust.
Failed catalytic converter: A failed catalytic converter can result in a lean condition, as the converter cannot convert the excess oxygen and hydrocarbons into harmless gases. Various factors, including overheating, contamination, or physical damage, can cause a failed catalytic converter. A failed catalytic converter can also produce a foul odor, restricted exhaust flow, or other emissions-related codes.
Electrical issues: Electrical issues can result in a lean condition, as the ECM/PCM cannot receive or process the signals from the sensors or actuators. Various factors, including wiring problems, connector issues, or failed components, can cause electrical issues. A thorough inspection of the electrical system and a diagnostic scan can identify any potential electrical issues that may cause P0171.
How to diagnose P0171?
Diagnosing P0171 can be challenging, as there are many possible causes and symptoms. However, the following steps can help to narrow down the possibilities and identify the root cause of P0171:
- The diagnostic trouble codes (DTCs) are retrieved using an OBD-II scanner or code reader. P0171 is a generic code that indicates a lean condition on Bank 1. Other codes or pending codes should be addressed first, as they may be related to or contribute to P0171.
- Perform a visual engine inspection, including the vacuum lines, hoses, gaskets, sensors, and electrical connections. Look for any signs of wear, damage, leaks, or contamination. Pay particular attention to the intake manifold gasket, PCV valve, brake booster, EVAP system, and air intake system.
- Check the fuel pressure using a fuel pressure gauge. The fuel pressure should be within the manufacturer’s specifications for the particular make and model of the car. If the fuel pressure is low, check the fuel pump, fuel filter, and fuel pressure regulator for any faults.
- Check the air intake system for any restrictions, including a dirty air filter, clogged air ducts, or a faulty air intake sensor. Replace or repair any faulty components as necessary.
- Check the exhaust system for leaks, including the exhaust manifold gasket, pipes, muffler, or catalytic converter. Replace or repair any faulty components as necessary.
- Check the operation of the sensors, including the O2S, MAF, MAP, and fuel injector. Use a scan tool or multimeter to measure and compare the sensor output to the manufacturer’s specifications. Replace or repair any faulty sensors as necessary.
- Perform a smoke test to locate any vacuum leaks that may be causing the lean condition. A smoke test can help to identify even the smallest leaks that may be difficult to detect visually.
How to fix P0171?
Fixing P0171 depends on the underlying cause of the lean condition. The following are some common fixes for P0171:
- Replace or repair any faulty sensors or components, including the O2S, MAF, MAP, fuel injector, air intake sensor, or exhaust system components.
- Clean or replace the air filter if it’s dirty or clogged.
- Check for any vacuum leaks and repair them if necessary.
- Repair or replace any damaged or leaking vacuum hoses or connections.
- Check the fuel pressure and replace the fuel filter or fuel pump if necessary.
- Repair or replace the catalytic converter if it’s damaged or failed.
- Check the ECM/PCM for any software updates or reprogramming that may address the lean condition.
- Reset the ECM/PCM and clear the codes using an OBD-II scanner or code reader. Test drive the car to confirm that the lean condition has been resolved.
- In some cases, fixing P0171 may require a combination of these fixes, depending on the underlying cause of the lean condition. It’s important to diagnose the root cause of P0171 before attempting any repairs, as misdiagnosis or improper repairs can lead to further damage or code recurrence.
P0171 is a common fault code that indicates a lean condition on Bank 1 of the engine. The symptoms of P0171 can include poor fuel economy, rough idling, or lack of power. The underlying causes of P0171 can include a faulty sensor, restricted air intake, exhaust leaks, failed catalytic converter, or electrical issues.
Diagnosing P0171 requires a thorough inspection of the engine, fuel system, air intake system, exhaust system, and sensors.
Fixing P0171 requires addressing the underlying cause of the lean condition, which may involve replacing or repairing faulty sensors or components, cleaning or replacing the air filter, checking for vacuum leaks, repairing or replacing damaged or leaking vacuum hoses or connections, checking fuel pressure, repairing or replacing the catalytic converter, updating or reprogramming the ECM/PCM, and clearing the codes.
About the Author
This article was created with the assistance of AI technology to aid the author, John Cunningham, who is a seasoned Red Seal-certified auto technician with more than 25 years of experience in vehicle repairs. However, please note that John Cunningham has edited the content to ensure accuracy and quality.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.