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P0171 and P0174 Lean Trouble Codes

What are P0171 and P0174 code definitions?

P0171 - System Too Lean Bank 1
P0174 - System Too Lean Bank 2

What Does P0171 and P0174 Mean?

DTCs P0171 and P0174 are generic codes that set when the feedback on the engine's air/fuel mixture becomes too lean for the Engine Control Module (ECM) to correct within its normal operating range. A lean mixture is a mixture that has too much air or not enough fuel. The ECM sets a target air/fuel ratio and monitors it using oxygen sensors and on newer vehicles air/fuel ratio sensors. When the ECM sees the engine running lean, it will add fuel and display the correction as a positive fuel trim percentage. If the correction goes past the ECMs predetermined range in the positive (add fuel) direction, the ECM will set a lean DTC P0171, P0174, or both.

P0171 and P0174 Symptoms

  • Check Engine Light
  • Low engine power
  • Poor idle
  • Stalling
  • Engine misfire
  • Spark/Engine Knock
  • Possible hard start or no start
    Tip : Vehicles can experience one or all these symptoms depending on severity of the condition

What Causes P0171 and P0174 to Set?

  • Vacuum leaks, intake air leaks: Vacuum leaks allow un-measured air to enter the engine, this air is unaccounted for and causes the engine to run lean. Cracked or broken vacuum lines, faulty intake gaskets and intake tubing are all common sources of vacuum leaks.
    Tip : Vacuum leaks are also commonly caused by components that use or regulate engine vacuum, such as purge solenoids, brake boosters and PCV valves
  • Incorrect air metering, Faulty Mass Air Flow (MAF) sensor: If the air entering the engine is measured incorrectly, more air can enter the engine than the ECM calculates for. An inadequate fuel charge will then be delivered, and a lean air/fuel mixture will be created. The leading cause is a malfunctioning MAF sensor, due to failure or being dirty.
    Tip : A faulty or skewed coolant temperature sensor and/or air temperature sensor can cause certain systems to mis-calculate air/fuel mixtures, resulting in P0171, P0174 DTCs.
  • Malfunctioning oxygen (O2) sensor or air fuel ratio (A/F) sensor: The ECM uses O2 sensors and A/F sensors to test the exhaust gas, this provides feedback on the air/fuel mixture delivered to the engine. If the sensors read excess oxygen in the exhaust, it tells the ECM the engine is running lean. Therefore, a faulty O2 sensor or A/F sensor can provide incorrect feedback to the ECM and create a false lean condition.
  • Low fuel pressure, incorrect fuel flow/delivery: Low fuel pressure or poor fuel flow causes less fuel to be delivered to the engine than commanded by the ECM, resulting in a lean running condition. This can be caused by several things such as a faulty fuel pump, dirty fuel filter or clogged fuel injectors.

How to diagnose P0171 and P0174 DTCs?

Recommended tools

  • Scan tool (preferably one that shows data PIDs)
  • Flashlight
  • Hand tools
  • Multimeter
  • MAF sensor cleaner
  • Soapy water spray bottle
  • Fuel pressure tester
  • Smoke machine

Diagnostic checks

Tip : See if any other DTCs are stored, additional trouble codes may help determine root cause of P0171, P0174 DTCs

  • Inspect for vacuum leaks: Visually inspect and listen for any obvious engine vacuum leaks, look for any broken or cracked vacuum lines, pay close attention to brake booster and PCV hoses. Check that air intake tubing is installed properly and is not torn or damaged. A smoke machine may be needed to locate hard to find leaks.
  • Check Mass Air Flow sensor: Remove the MAF sensor and look for signs of contamination, debris that makes it past the air filter or oil from an aftermarket air filter can stick on MAF sensor and cause inaccurate signals. If dirty, clean sensor using a quality MAF cleaner. If a scan tool is available, check MAF sensor signal reading and compare to vehicles listed specification.
  • Inspect Exhaust: Leaking exhaust can affect O2 sensor readings. Visually inspect and listen for exhaust system leaks. Look for any cracks or damage to exhaust pipes and any black soot marks. These are tell-tale signs of a leaking exhaust. For hard-to-find leaks, spray soapy water on suspected locations and look for bubbles to pinpoint leak.
    Tip : Exhaust leaks behind the rear O2 sensor will not affect air/fuel mixture
  • Check oxygen sensors and/or air fuel ratio sensors: Using a scan tool, monitor the signal voltage of the O2 sensors and/or A/F sensors and make sure they are operating correctly. If a scan tool is not available a multimeter can be used to read signal voltage. Every vehicle operates differently so you should consult your vehicles service information to obtain the correct procedure in checking these signals.
  • Check fuel pressure: Obtain a fuel pressure reading and compare to the factory listed specification. Some vehicles are equipped with a fuel pressure sensor and fuel pressure can be read using a scan tool. If your vehicle does not have this option, a fuel pressure gauge will have to be installed to read fuel pressure manually. Be careful, as many modern vehicles use a gasoline direct injection (GDI) system and run at extremely high pressures. So be sure to check service information for the correct procedure.

How do I repair a P0171 or P0174 DTC?

Due to the number of components that are involved in the operation of your vehicles engine management system. Proper and accurate diagnosis are required to pin-point the cause of P0171 and P0174 diagnostic trouble codes. As listed above there are several systems involved that could potentially be malfunctioning, so guessing could become costly.

For more information, questions, or help diagnosing your vehicle, contact the GoTech technical support team via phone or chat below.