Causes and fixes for a Toyota P0171 Code: "Fuel Trim System Too Lean Bank 1".

Triggered by the first exhaust oxygen sensor (upstream) on the "Bank 1" side of the engine. Problematic code since it can be caused by several things.

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Possible Causes of a Toyota P0171 Code:
Failing Mass Air Flow Sensor
Clogged Fuel Filter
Bad Fuel Pressure Regulator
A Weak or Failing Fuel Pump
Vacuum Leak
Bad Oxygen Sensor
Bad Injector

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