Circuit Description

The engine control module (ECM) 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 ECM determines fuel delivery based on sensor signals, without oxygen sensor input. During Closed Loop, the oxygen sensor inputs are added and used by the ECM to calculate fuel trim. If the oxygen sensors indicate a lean condition, fuel trim values will be above 0 percent. If the oxygen sensors indicate a rich condition, fuel trim values will be below 0 percent. Short Term Fuel Trim (FT) values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long Term FT values change slower in response to trends in Short Term FT adjustments. The fuel trim 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. If the ECM detects excessive vapor then a pass is logged. This DTC is continuously monitored during operation. If the ECM detects an excessively rich condition, DTC P0172 or P0175 sets.