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P0171: System too lean Bank 1

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What is P0171?

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 a lean fuel mixture on Bank 1 of a V-type engine.

A P0171 code indicating a lean mixture is relatively common. 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 fuel for gasoline.

Lesser fuel can lead to friction between the moving parts of your engine and can damage critical components.

Common symptoms

If your vehicle has a faulty fuel delivery system, 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
  • Engine runs and dies
  • Tip of spark plugs turn white
  • Catalytic converter damage (if the PCM experiences a prolonged fuel trim)

Can I still drive?

Yes. The P0171 lean code is pretty 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 have the P0171 system code diagnosed at the earliest.

P0171 causes

Several factors could trigger an engine light for this OBD-II trouble code, including faults in the fuel system. Some of the common causes 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
  • Faulty or dirty fuel injectors
  • Faulty air fuel ratio sensor
  • Faulty oxygen sensor (02 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 perform the diagnosis and repair for you, whether you own a Toyota, Ford, or Mercedes.

However, to give you an idea here’s what a mechanic will typically do to fix this engine code: 

  • Check for other codes: Use the scan tool to ensure that no other engine code exists.

Assuming that no other engine code displays on the scan tool, your mechanic may diagnose code P0171 by:

  • Inspecting vacuum lines: Checking the PCV valve and all vacuum hoses and lines for any air leak in the intake manifold. This involves inspecting the vacuum line, PCV valve, intake gasket, and intake manifold.
  • Running tests on the Mass Air Flow Sensor: Using the manufacturer-recommended procedure, check the intake boot from the MAF sensor to the throttle body. Replace the air filter and also clean the MAF sensor.
  • Inspecting the exhaust system: Checking for any exhaust leak or unmetered air before checking the oxygen sensor. 
  • Using a fuel pressure gauge to check the fuel pressure: A low fuel pressure could affect the fuel trim. They’ll inspect the fuel injectors, fuel filter, fuel pump, and fuel pressure regulator to determine the source of low fuel pressure. 
  • Replacing air fuel and/or O2 sensor: The mechanic will inspect and replace the faulty O2 sensor if the code persists.
  • Inspecting and replacing spark plugs: After fixing the code, they’ll inspect the spark plugs for any white haze or crust on the tips. If so, they’ll replace all the spark plugs.

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 is diagnosing what has caused the engine to run lean.

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 perform the diagnosis and repair for you, whether you own a Toyota, Ford, or Mercedes.

However, to give you an idea here’s what a mechanic will typically do to fix this engine code:

  • Check for other codes: Use the scan tool to ensure that no other engine code exists.
  • Inspect vacuum lines: Check the PCV valve and all vacuum hoses and lines for any air leak in the intake manifold. 
  • Inspect the Mass Air Flow Sensor: Check the intake boot from the MAF sensor to the throttle body. Replace the air filter and also clean the MAF sensor.
  • Inspect the exhaust system: Check for any exhaust leak or unmetered air before checking the oxygen sensor. 
  • Check the fuel pressure: A low fuel pressure could affect the fuel trim. They’ll inspect the fuel injectors, fuel filter, fuel pump, and fuel pressure regulator to determine the source of low fuel pressure. 
  • Replace air-fuel and/or O2 sensor: The mechanic will inspect and replace the faulty O2 sensor if the code persists.
  • Inspect and replace spark plugs: After fixing the code, they’ll inspect the spark plugs for any white haze or crust on the tips. If so, they’ll replace all the spark plugs.

Fixing the P0171 code 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.)

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

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

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