Circuit Description

WARNING: This page is about a different car, the 2002 Pontiac Grand Am, 2002 Oldsmobile Alero, 2002 Chevrolet Monte Carlo, and 2002 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.

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 Oxygen Sensor (O2S) input. During Closed Loop, the oxygen sensor inputs are added and used by the PCM to calculate short and long term fuel trim, delivery adjustments. If the O2S indicate a lean condition, the fuel trim values will be greater than 0 percent. If the O2S indicate a rich condition, the fuel trim values will be less than 0 percent. The short term fuel trim values change rapidly in response to the Heated Oxygen Sensors (HO2S) voltage signals. The long term fuel trim makes coarse adjustments in order to maintain air/fuel ratio of 14.7:1. 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 PCM detects an excessively rich condition, this DTC will set. If the PCM detects excessive vapor then a pass is logged.