Code Definition

P0171 — System Too Lean Bank 1

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Toyota

About P0171

P0171 is a generic (obd-ii standard) powertrain diagnostic trouble code recorded for Toyota vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0171 is stored, the module has detected a bank-specific fault, meaning it applies only to one cylinder bank of a multi-bank engine.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0171 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The P0171 fault code is set when the Engine Control Module (ECM) detects that the air-fuel mixture in Bank 1 of the engine is too lean, meaning there is more air than the ideal proportion of fuel. This condition is usually identified by the ECM through the oxygen sensors, which detect a higher than expected level of oxygen in the exhaust. A lean mixture can result from various factors, including insufficient fuel delivery, excessive air intake, or sensor malfunctions. This imbalance can lead to engine performance issues, such as misfiring, rough idle, or lack of power, as well as increased emi...

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