P0171: System Too Lean (Bank 1)

What Is P0171?

Code P0171 is a generic OBD-II diagnostic trouble code (DTC) defined as P0171 System too lean Bank 1.

It’s a trouble code that your vehicle’s Powertrain Control Module (PCM) or Engine Control Module (ECM) logs and alerts via Check Engine Light when it detects an incorrect lean fuel mixture on Bank 1 of a V-type engine.

A P0171 code indicating a lean mixture is relatively standard. This trouble code is also considered generic as it applies to all vehicle makes and models.

What Is a Lean Air Fuel Mixture?

An air fuel mixture containing more air or less fuel than the standard stoichiometric ratio is regarded as a lean fuel mixture. This ratio is 14.7 of air to 1 part of the fuel for gasoline engines.

Code P0171 indicates that the car’s computer has incorrectly calculated the fuel trim values essential for maintaining a balanced airfuel mixture in the engine. 

Lesser fuel can lead to friction between the moving parts of your engine and can damage critical components, leading to the check engine light blinking non-stop.

Common Symptoms

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

• Lack of power or low engine speed
• Check Engine Light comes on or flashes
• Rough idle
• Hesitation or stumble from the engine
• Misfires
• The engine runs and dies
• The tip of a spark plug turns white
• Catalytic converter damage (if the PCM experiences a prolonged fuel trim)

Can I Still Drive?

While driving with this code is technically possible, doing so for extended periods is not recommended. The P0171 lean code is severe as it relates to the fuel system, and you should address it ASAP.

Since a faulty fuel delivery triggers the P0171 code, your engine will not maintain the correct air fuel ratio. This could lead to fuel wastage and reduced mileage. 

You also run the risk of low engine speed. Your car could overheat, leading to irreversible engine damage.

Moreover, if you continue to drive with a faulty engine for too long, you could damage other critical parts of your vehicle, such as the spark plugs, cylinder pistons, and the catalytic converter. Some of these repairs are pretty costly too.

To avoid all this, it’s best to diagnose the P0171 system code at the earliest.

P0171 Causes

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:

• A faulty fuel pressure regulator
• A weak fuel pump
• A clogged fuel filter
• A vacuum leak from the PCV valve, vacuum line, or intake manifold gasket
• Defective or dirty fuel injectors
• Faulty air fuel ratio sensor (AF sensor)
• Faulty oxygen sensor (O2 sensor)
• A clogged or faulty mass air flow sensor (MAF sensor)
• Faulty MAP sensor (manifold absolute pressure)
• Exhaust leak or unmetered air entering the engine
• A faulty Powertrain Control Module

Diagnosis

Since fixing the P0171 code requires specialized equipment and knowledge, doing it as a DIY repair isn’t recommended. Ideally, you should have a certified technician diagnose and fix the code for you.

However, to give you an idea, here’s what a mechanic will typically do to diagnose issues related to the DTC P0171 engine code:

1. Check for other codes
Use the scan tool to ensure no other engine code exists. Assuming that no other fault code displays on the scan tool, code P0171 may be diagnosed with the next steps.

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

3. Run tests on the Mass Air Flow sensor
Using the manufacturer-recommended procedure, check the intake boot from the MAF sensor to the throttle body. 

4. Inspect the exhaust system
Check for any exhaust leak or unmetered air before checking the oxygen sensor. 

5. Use a fuel pressure sensor gauge to check the fuel pressure
A low fuel pressure could affect the fuel trim. If the fuel pressure sensor gauge indicates an issue, inspect the fuel injector system, fuel filter, fuel pump, and fuel pressure regulator to determine the source of low fuel pressure. 

6. Evaluate the air fuel and/or O2 sensor
Inspect O2 sensor or AF sensor for faulty readings if the code persists.

7. Inspect spark plugs
After fixing the code, examine the plugs for any white haze or crust on the tips. 

If none of the tests shows any problems, the issue could be with the Powertrain Control Module.

Possible Repairs For P0171 & Costs

The first step to fixing the P0171 engine code is diagnosing what has caused the engine to run lean.

Fixing the P0171 code and removing the check engine light begins with an hour of diagnosing time. Depending on the labor cost in your area, this can range from $125-$150.

Once the issue is diagnosed, the auto repair shop will provide a repair estimate (including part cost and labor charges.)

Depending on the issue, the mechanic will attempt the following:

➤ Mend vacuum lines: The mechanic will fix vacuum leaks due to a bad PCV valve or any leaking vacuum hose. They’ll also replace a worn intake manifold gasket, if required.

➤ Clean or replace the Mass Air Flow sensor: The mechanic will look for dirt accumulation in the MAF sensor and throttle body. Likely, they’ll replace the air filter and clean the MAF sensor using a specialized MAF cleaner.

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

➤ Calibrate the fuel pressure: As the issue of low fuel pressure could be caused by a faulty fuel injector system, fuel filter, fuel pump, or fuel pressure regulator, your mechanic will take numerous courses of action, such as cleaning the fuel filter or replacing the fuel pump.

➤ Replace air fuel and/or O2 sensor: The mechanic will inspect and replace the faulty O2 sensor if the P0171 engine code persists.

➤ Replace any spark plug: Your mechanic will replace any spark plug in your vehicle if they spot any white haze or crust on the tips.

Here’s generally what you can expect to pay for different repairs:

• Cleaning dirty fuel injectors: $50-$80 
• Fixing a vacuum leak: $100-$200
• Exhaust leak repair: $100-$200
• Cleaning the mass air flow sensor: $100
• Replacing the mass air flow sensor: $300
• Fuel pressure regulator replacement: $200-$400
• Air fuel sensor or oxygen sensor: $200-$400
• Fuel pump replacement: $1300-$1700