A P0171 code is set by your vehicle’s powertrain control module (PCM) when it detects that the air-to-fuel ratio of your vehicle is too lean. This means that the PCM is relaying that the engine is receiving too much air into the combustion chamber and not enough fuel.
The PCM receives signals from the oxygen sensors located in your vehicle’s engine bay and is constantly working to adjust the right amount of air and fuel based on the readings provided by these sensors.
When the factory predefined range falls out of line, more specifically, is lower in fuel than the motor needs, the PCM will trigger the P0171 code. The PCM then calls for more fuel to be sent through the fuel injectors and into the combustion chamber of the engine.
This action is called a fuel trim, and in the case of a P0171 code, where more fuel is called for, this is a positive fuel trim.
What Does the P0171 Trouble Code Mean?
The P0171 code stands for “System Too Lean (Bank 1).” Bank 1 refers to the side of the engine containing cylinder 1, and “too lean” means the air-fuel mixture has too much air and insufficient fuel.
The ECM constantly monitors this balance using data from the oxygen sensors in the exhaust system. If the sensors detect that the air-fuel ratio deviates significantly from the ideal ratio of approximately 14.7:1 for an extended period, the ECM triggers the P0171 code.
Why Does a P0171 Code and Running “Too Lean” Matter?
“Lean” running cars and light trucks are highly polluting vehicles. Most of the NOx pollution, which is poisonous and can cause asthma, is caused by vehicles that are running too lean.
A lean running car can also misfire, which puts raw fuel (HCs) into the catalytic converter which can result in internal damage, and on into the atmosphere. When you are behind a car or truck that is misfiring it makes your eyes burn.
In comparison, a “rich” running engine (one that is not misfiring as a result) has no smell (CO is odorless) or you may detect a rotten egg smell, which is sulfur dioxide produced by the Catalytic Converter.
P0171 is not an Oxygen Sensor problem. Before a P0171 code is possible, the computer first ran a series of tests to validate the readings from the oxygen sensors.
Since the oxygen sensors passed their readiness tests and didn’t set any codes, the computer then looked to the Fuel Trim adjustment. When the computer determined the air-to-fuel mixture to be too lean, it then set the P0171 code.
Symptoms of the P0171 Code
When the P0171 code is set, you may experience one or more of the following symptoms:
- Check Engine Light (CEL): The most obvious sign is the illumination of the check engine light on the dashboard.
- Reduced Engine Performance: You might notice a decrease in power, especially during acceleration.
- Rough Idle: The engine may idle unevenly or feel shaky.
- Stalling: In severe cases, the engine might stall, particularly at low speeds or when coming to a stop.
- Increased Fuel Consumption: Ironically, despite the lean condition, the ECM may overcompensate by injecting more fuel, leading to poor fuel efficiency.
- Engine Hesitation: The vehicle may hesitate or jerk during acceleration.
Common Causes of the P0171 Code
Several factors can lead to a lean condition in Bank 1. Identifying the root cause is essential for resolving the issue effectively. Here are the most common causes:
- Vacuum Leaks: Cracked or disconnected vacuum hoses, Faulty intake manifold gaskets, Leaking throttle body or EGR valve.
- Faulty Mass Air Flow (MAF) Sensor: A dirty or malfunctioning MAF sensor can send incorrect data to the ECM, resulting in an improper air-fuel mixture.
- Clogged or Dirty Fuel Injectors: Restricted fuel injectors can reduce the amount of fuel delivered to the engine.
- Weak Fuel Pump or Clogged Fuel Filter: Insufficient fuel pressure due to a failing fuel pump or a clogged filter can lead to a lean condition.
- Faulty Oxygen Sensor: A malfunctioning upstream oxygen sensor may send inaccurate readings to the ECM.
- Exhaust Leaks: Leaks in the exhaust system near the oxygen sensor can cause erroneous lean readings.
- PCV System Issues: A defective Positive Crankcase Ventilation (PCV) valve can introduce unmetered air into the intake manifold.
- Software Issues: In rare cases, outdated or corrupted ECM software may misinterpret sensor data.
Diagnosing the P0171 Code
It’s vital to perform an OBD-II scan of your vehicle before assuming the issue’s cause. You or a mechanic should scan for Code P0171 to ensure this is the correct diagnosis for your automobile and that no other fault codes are showing up upon seeing a check engine light.
You might think that changing out some sensors will resolve the issue, but a proper OBD-II scan will confirm the problem takes place around Bank 1.
It is recommended that someone with experience in the diagnostics field check for Code P0171. A professional mechanic should have some additional tools to get to the bottom of the issue.
Below is a list of areas to inspect after finding Code P0171. Complete these steps in the order until you uncover the reason for the error message:
- Use a vacuum gauge to locate leaks —only when the engine is off.
- Check fuel sensor and mass flow air sensor performance with a fuel pressure gauge.
- Follow manufacturer instructions for testing oxygen sensors throughout the fuel system.
- Once you’ve tried all other options, the problem likely lies in the power control module.
Fixing the P0171 Code
The solution for a P0171 code depends on the underlying cause. Here are some common fixes:
- Repair Vacuum Leaks: Replace damaged vacuum hoses and faulty intake manifold gaskets. Tighten loose connections.
- Clean or Replace the MAF Sensor: If cleaning the MAF sensor doesn’t resolve the issue, replace it with a new one.
- Service the Fuel System: Clean or replace clogged fuel injectors, replace the fuel filter, or repair a failing fuel pump.
- Replace Faulty Oxygen Sensors: Install new oxygen sensors if the old ones are providing incorrect data.
- Fix Exhaust Leaks: Seal any exhaust leaks near the oxygen sensor.
- Replace the PCV Valve: Install a new PCV valve if the existing one is defective.
- Update ECM Software: Consult your dealership or a qualified mechanic to update the ECM software if necessary.
Preventing the P0171 Code
Regular maintenance can help prevent the P0171 code and other engine issues. Here are some tips:
- Keep Sensors Clean: Regularly clean or replace the MAF sensor and oxygen sensors as recommended by your vehicle’s manufacturer.
- Inspect Vacuum Hoses: Periodically check vacuum hoses for wear and tear.
- Maintain the Fuel System: Use high-quality fuel and add fuel system cleaners periodically to prevent clogs.
- Replace the Air Filter: Change the air filter at regular intervals to ensure proper airflow to the engine.
- Monitor Performance: Pay attention to changes in engine performance and address issues promptly.
What is the cost to resolve a P0171 code?
The cost to repair a P0171 code can vary depending on the issue. Some repair estimates include:
- Vacuum leak: $100–$200
- Mass airflow sensor (MAF): $300
- Fuel pump: $1300–$1700
- Fuel pressure regulator: $200–$400
- Oxygen sensor: $200–$300
- Exhaust repair: $100–$200
This code will require a diagnostic check, which will run about $100 depending on the time it takes to find the issue. As there is a laundry list of reasons why the engine runs lean, the average price per repair will vary greatly.
You may get lucky with one of the simple repairs mentioned above. A broken vacuum hose sucking in air can be around $50, or a clogged fuel filter for $100.
On the other hand, an oxygen sensor or mass air flow sensor can bring a bill of $400 to $500 dollars. A bad fuel pump can be really expensive costing around $600 to $800.
If you vehicle is throwing this code, schedule a diagnostic check with a mechanic immediately.