DTC P0171: Fuel Trim System Lean: Description

WARNING: This page is about a different car, the 2002 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.

Exhaust Oxygen Sensor (O2S) is used to determine oxygen content of exhaust. Oxygen content indicates when engine is operating lean or rich. When engine is operating lean, exhaust will have more oxygen content. Once at operating temperature, O2S will produce a voltage in relation to oxygen content of exhaust. O2S requires an operating temperature of 600°F (360°C) to produce voltage. O2S will produce more voltage when exhaust is rich and less voltage when exhaust is lean. O2S operating voltage range is 0.10-1 volt (100-1000 mV).

Alternate Fuel Engine Control Module (AF ECM) uses closed loop air/fuel metering system when O2S is at operating temperature. During normal closed loop fuel control operation, AF ECM will add fuel (enriches mixture) when O2S indicates lean exhaust content. When O2S indicates rich exhaust content, AF ECM will subtract fuel (lean out mixture). This addition and subtraction to air/fuel mixture may be indicated on scan tool as long and short term fuel trim values. Short term values are adjustments to fuel mixture that occur quickly. Long term values are adjustments to fuel mixture that accumulate over longer periods of time. Long and short term fuel trim values are displayed on scan tool as percent. Positive percentage values indicate fuel is being added and negative percentage values indicate fuel is being subtracted from the mixture.

This DTC is used to indicate a fully rich fuel trim system that cannot compensate for a lean engine exhaust content.