P0171 - System Too Lean (Bank 1)
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0171 indicates that the Engine Control Unit (ECU) has reached its maximum limit for positive fuel trim correction (Long Term Fuel Trim - LTFT). This occurs when the oxygen sensor (Lambda) detects an excess of oxygen relative to fuel. To maintain the stoichiometric ratio (Lambda 1.0), the ECU increases the injector pulse width. When the correction exceeds approx. +20-25% for a calibrated duration, the code is set. This is typically caused by 'unmetered air' entering the intake after the MAF sensor or an insufficient fuel supply.
Possible Causes
Vacuum leak/Unmetered air (intake manifold gasket, cracked hoses, or PCV system)
Dirty or faulty Mass Air Flow (MAF) sensor
Clogged fuel filter or low fuel pressure/faulty pump
Faulty upstream oxygen sensor (Lambda) providing false lean readings
Symptoms
Recommended Solution
Workflow
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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