P0171/P0174 System Too Lean Diagnostic Aids

The ability to identify the type of lean condition causing the concern can be crucial to a correct diagnosis. The following are possible causes:

• Air Measurement System
  
  With this condition, engine may actually run rich or lean of stoichiometry (14.7:1 air/fuel ratio), if Powertrain Control Module (PCM) is not able to compensate enough to correct for condition. One possibility is that the mass of air entering engine is actually more than what MAF sensor is indicating to PCM. Examples; with a contaminated MAF sensor, engine would run lean at higher RPM because PCM would deliver fuel for less air than is actually entering engine. Examples are as follows; inaccurate MAF sensor measurement can be caused by a corroded connector, contaminated or dirty MAF. A contaminated MAF sensor will typically result in a rich system at low airflows resulting in PCM reducing fuel, and a lean system at high airflows resulting in PCM increasing fuel, etc.
• Vacuum Leaks/Unmetered Air
  
  With this condition, engine may actually run lean of stoichiometry (14.7:1 air/fuel ratio), if PCM is not able to compensate enough to correct for condition. This condition can be caused by unmetered air entering engine, or due to a MAF malfunction. Volume of air entering engine is actually more than what MAF sensor is indicating to PCM. Vacuum leaks will normally be most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates that fault occurred at idle, a check for vacuum leaks/unmetered air might be the best starting point. Examples of this are: loose, leaking or disconnected vacuum lines, intake manifold gaskets or "O" rings, throttle body gaskets, brake booster, air inlet tube, stuck/frozen/aftermarket PCV valve or an unseated engine oil dipstick, etc.
• Insufficient Fueling
  
  With this condition, engine may actually run lean of stoichiometry (14.7:1 air/fuel ratio), if PCM is not able to compensate enough to correct for condition. This condition can be caused by a fuel delivery system concern that restricts or limits amount of fuel being delivered to engine. This condition will normally be most apparent when engine is under a heavy load and at high RPM, when a higher volume of fuel is required. If freeze frame data indicates that fault occurred under a heavy load and at higher RPM, a check of fuel delivery system, such as checking fuel pressure with engine under a load might be the best starting point. Examples of this are: low fuel pressure (fuel pump, fuel filter, fuel leaks, restricted fuel supply lines), fuel injector concerns, etc.
• Exhaust System Leaks
  
  In this type of condition, engine may actually be running rich of stoichiometry (14.7:1 air/fuel ratio), because fuel control system is adding fuel to compensate for a perceived lean condition. This condition is caused by oxygen entering exhaust system from an external source. The HO2S will react to this exhaust leak by increasing fuel delivery. This condition will cause exhaust gas mixture from cylinder to be rich. Examples of this are: exhaust system leaks upstream or near HO2S, poorly welded/leaking HO2S boss or malfunctioning secondary air injection system, etc.