The Fuel Composition Sensor (FCS) circuit monitors the number of ethanol in the gasoline in a flex-fuel engine. The ethanol may fill the fuel tank at some point, requiring proper adjustment for optimum performance and fuel economy. This Fuel Composition Sensor circuit sends a signal to the ECM (Engine Control Module) based on the ethanol level. The ECM then adjusts the ignition timing and fuel injector pulse width, resulting in a more efficient fuel burn.
P0178 OBD2 diagnostic code is configured when the ECM determines a low voltage within the Fuel Composition Sensor circuit, it is normal when this DTC is present that the source of the problem is electrical.