Mechanics

Lean Exhaust Code Repair P0171 P0174

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Step by step guide on how to repair automobile lean code problems, this article pertains to most vehicles.

Difficulty Scale: 4 of 10

Tools and Supplies Needed

  • Code Reader
  • Flashlight
  • Wrench set
  • Socket Set
  • Protective gloves and eyewear

Begin with the vehicle parked on flat ground in park with the emergency brake set.

Step 1 - Once a check engine or service engine soon light has determined a lean engine code has materialized, a repair procedure inspection is needed. Explanation: A lean code is enabled when the computer has detected enrichment adjustments have maxed out, with no results, this inability triggers the lean code.


Lean Mixture Code

Step 2 - Unmetered air which enters the intake system of the engine can cause a lean mixture code to be triggered, inspect all vacuum lines for cracks or dilapidation. Check the integrity of vacuum hoses at each end of the hose.


Vacuum Leak

Step 3 - The intake system utilizes gaskets which seal various components, if these gaskets fail a vacuum leak will result, causing the code failure. Use carburetor cleaner to help locate any potential leaks by spraying around gasket surfaces and observe a rise in engine idle speed if a leak exists.


Vacuum Leak Testing

Step 4 - An exhaust leak will allow fresh air to enter the system on the negative pressure side of an exhaust event, this will cause an oxygen sensor to produce a lean reading. An exhaust system manages both pressure and vacuum cycles while an engine runs. To inspect for an exhaust leak, have a helper start the engine and hold the idle at about 1500 rpm, listen for additional exhaust noises coming from the system including manifolds and head pipes. Also, look for black soot at any point in the system as this can indicate an exhaust leak.


Exhaust leak

Step 5 - After miles of driving, the mass air flow sensor's filament or hot wire, can become contaminated due air impurities which create false readings. Using sensor safe carburetor cleaner to clean this filament can sometimes remove these deposits allowing the sensor to obtain a correct reading.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2014-07-07)