DTC P0172: Fuel Trim System Rich: Description

WARNING: This page is about a different car, the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, and 2002 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.

The Heated Exhaust Oxygen Sensor (HO2S) 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, HO2S will produce a voltage in relation to oxygen content of exhaust. HO2S requires an operating temperature of 600°F (360°C) to produce voltage. HO2S will produce more voltage when exhaust is rich and less voltage when exhaust is lean. HO2S operating voltage range is 0.10-1.00 volt (100-1000 mV).

Alternate Fuel Engine Control Module (AF ECM) uses closed loop air/fuel metering system when HO2S is at operating temperature. During normal closed loop fuel control operation, AF ECM will add fuel (enriches mixture) when HO2S indicates lean exhaust content. When HO2S 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 lean fuel trim system that cannot compensate for a rich engine exhaust content.