Circuit Description

The powertrain control module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop the PCM determines fuel delivery based on sensor signals without heated oxygen sensor (HO2S) input. During Closed Loop the PCM adds HO2S inputs and level of purge to calculate Short and Long Term fuel trim adjustments. If the HO2S indicate a lean condition, fuel trim values will be above 0 percent. If the HO2S indicate a rich condition, fuel trim values will be below 0 percent. The values for the Short Term fuel trim change rapidly in response to the HO2S voltage signals. Long Term fuel trim makes coarse adjustments in order to maintain an Air/Fuel Ratio of 14.7:1. A block of cells contain information arranged in combinations of engine speed (RPM) and engine load for a full range of vehicle operating conditions. The long term fuel trim diagnostic is based on an average of cells currently being used, but is capable of identifying a lean condition within a single cell. The PCM determines the cell based on the current engine speed and engine load. If the PCM detects an excessively lean condition, DTC P0171 sets.