Circuit Description

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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 Loop and Closed Loop (CL). During Open Loop, the PCM determines fuel delivery based on sensor signals, without heated oxygen sensor (HO2S) input. During CL, the PCM adds HO2S inputs and level of purge to calculate the short and long term fuel trim (FT) adjustments. If the HO2S indicate a lean condition, the FT values will be above 0 percent. If the HO2S indicate a rich condition, the FT values will be below 0 percent. The short term FT values change rapidly in response to the HO2S voltage signals. The long term FT makes coarse adjustments in order to maintain Air/Fuel Ratio of 14.7:1. The FT diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition. A block of cells contain information arranged in combinations of engine RPM and engine load for a full range of vehicle operating conditions. The long term FT diagnostic is based on an average of cells currently being used. The PCM selects the cells based on the engine speed and load. If the PCM detects an excessively rich condition, DTC P0172 sets.