P0171 Code: System Too Lean (Bank 1)

What Is P0171?

Code P0171 is a generic OBD-II diagnostic trouble code (DTC) that stands for “System Too Lean (Bank 1).” Since it’s a generic code, it applies to most vehicle makes and models.

Your vehicle’s Powertrain Control Module (PCM) or Engine Control Unit (ECU) sets this code when it detects a lean air-fuel mixture on Bank 1 and triggers the Check Engine Light.

Now you must be wondering: What is a lean air-fuel mixture?

A lean fuel mixture means the engine has more air and less fuel than normal. For gasoline engines, the ideal mix is about 14.7 parts of air to 1 part of fuel. If there’s more air than that, the mixture is considered “lean.”

As a result, code P0171 indicates that the car’s computer is struggling to maintain the correct air-fuel mixture, likely due to incorrect adjustments.

Insufficient fuel can cause friction between the moving parts of your engine and damage critical components, causing the Check Engine Light to blink nonstop.

10 Common Symptoms of DTC P0171

If your vehicle has a faulty fuel delivery system and the P0171 diagnostic trouble code appears, you’re likely to encounter one or more of the following symptoms:

1. A lit or flashing Check Engine Light
2. Hesitation or stumble from the engine
3. Rough idle
4. Lack of power or engine stalling
5. Misfires
6. The engine runs and dies
7. Pinging, knocking, or hissing sounds from the engine bay
8. Black smoke from the exhaust system
9. The tips of the spark plugs turn white
10. Catalytic converter damage (if the PCM experiences a prolonged fuel trim)

11 Underlying Causes That Trigger Code P0171

Several factors could trigger the Check Engine Light for this OBD-II trouble code, including faults in the fuel system. Some of the common causes for the DTC P0171 fault code include:

1. A faulty fuel pressure regulator
2. A weak fuel pump
3. A clogged fuel filter
4. A vacuum leak from the Positive Crankcase Ventilation (PCV) valve, vacuum line, or intake manifold gasket
5. Defective or clogged fuel injector
6. Faulty air-fuel sensor (AF sensor)
7. Faulty oxygen sensor (O2 sensor)
8. A clogged or faulty mass air flow sensor (MAF sensor)
9. Faulty MAP sensor (manifold absolute pressure)
10. Exhaust leak or unmetered air entering the engine
11. A faulty PCM

Is Code P0171 Serious & Can You Still Drive?

The P0171 code is considered serious because it involves the fuel system, so you should address it ASAP.

And while the vehicle may still be drivable, it’s not recommended to continue doing so with this code for an extended period.

Here’s why:

Since a faulty fuel delivery triggers the P0171 error code, your engine won’t maintain the correct air-fuel ratio, which could lead to fuel wastage and reduced mileage. You may also experience engine performance issues like rough idle, low speed, or overheating, which can lead to irreversible damage. 

Driving with insufficient fuel or a lean condition for too long can also harm components, such as spark plugs, pistons, and the catalytic converter, requiring expensive repairs.

To avoid all this, it’s best to find and fix the root cause of the P0171 system code as soon as possible.

How to Diagnose Error Code P0171: A General Know-How

Since fixing the P0171 code requires specialized equipment and knowledge, we don’t recommend a DIY diagnosis or repair. It’s best to have a certified technician fix the code for you.

But, to give you an idea, here’s how a mechanic will typically proceed: 

1. Check for other codes: Use the scan tool to check for other engine codes and fix them first. If no other fault code is displayed on the scan tool, the mechanic will proceed to the next step.

2. Inspect vacuum lines: Check the PCV valve and all vacuum hose components in the intake manifold for leaks. Also, check the intake gasket for leaks.

3. Test the mass airflow sensor: Use a scan tool or multimeter to check whether the MAF sensor accurately measures airflow. The mechanic may also inspect the intake boot between the MAF sensor and throttle body for cracks or leaks, as these can affect sensor performance.

4. Inspect the exhaust system: Check for any exhaust leaks or unmetered air.

5. Evaluate the air-fuel ratio (AF) and O2 sensors: If the P0171 error code persists, inspect the oxygen and the air-fuel sensor for incorrect readings. These sensors help regulate the air-fuel mixture, and if they send inaccurate data, the engine may run lean.

6. Check the fuel pressure: Use a fuel pressure sensor gauge to check for low fuel pressure. Then, inspect the fuel injectors, fuel filter, fuel pump, and fuel pressure regulator to determine the cause.

7. Inspect spark plugs: Check the spark plugs for signs of a lean condition, such as white deposits or a chalky residue on the tips, indicating improper combustion.

8. Check the Powertrain Control Module (PCM): If the mechanic finds no faults elsewhere, they may update the PCM software or test it for other issues.

What Other Related Codes Can Occur Alongside P0171?

Here are some other codes that may appear alongside P0171:

• P0170: Fuel trim malfunction
• P0174: System too lean (Bank 2)
• P0300–P0312: Random or cylinder-specific misfires
• P0101–P0104: Mass airflow (MAF) sensor problems
• P0130–P0135: Oxygen sensor circuit faults

Possible Repairs for P0171 & Costs

Fixing the P0171 code and clearing the Check Engine Light typically requires an hour of diagnostic work. Once the mechanic identifies the root cause, they’ll provide a price estimate that includes parts and labor.

Depending on the issue, the mechanic may perform the following repairs:

• Address low fuel pressure issues: Low fuel pressure could result from faulty fuel injectors or other parts like the fuel filter, fuel pump, or fuel pressure regulator. Your mechanic may take various steps to resolve the issue, such as cleaning the filter or replacing the pump.

• Repair vacuum lines: The mechanic will fix leaks caused by a bad PCV valve or a leaking PCV hose. They’ll also replace a worn intake manifold gasket if necessary.

• Clean or replace the mass airflow sensor: The mechanic will look for dirt accumulation in the MAF sensor and throttle body. They’ll likely replace the air filter and clean the MAF sensor using a specialized MAF cleaner.

• Replace the O2 sensor: If the P0171 engine code persists, the mechanic will inspect and replace the faulty oxygen sensor.

• Replace bad spark plugs: The mechanic will replace your vehicle’s spark plugs if they spot any white haze or crust on the tips.

• Fix exhaust system issues: If your mechanic detects an exhaust leak or unmetered air, they’ll weld a new section of pipe onto the component to ensure the exhaust system is leakproof.

Here’s what you can expect to pay for all these repairs:

• Vacuum leak repairs: $12 to $1250
• Clogged fuel injector cleaning: $60 to $100 
• Mass air flow sensor replacement: $250 to $470
• Fuel pressure regulator replacement: $260 to $870
• Exhaust leak repair: $265 to $4170
• Oxygen sensor replacement: $360 to $520
• Fuel pump replacement: $1100 to $1300