P0178 Buick Fuel Composition Sensor Circuit Low Frequency
– Fuel Composition Sensor harness is open or shorted
– Fuel Composition Sensor circuit poor electrical connection
The fuel composition sensor (FCS), or flex fuel sensor (service parts term), measures the ethanol-gasoline ratio of the fuel being used in a flexible fuel vehicle. Flexible fuel vehicles can be operated with a blend of up to 85 percent ethanol. In order to adjust the ignition timing and the air to fuel ratio, the engine management system requires information about the percentage of ethanol in the fuel.
The FCS uses an incoming fuel connection and an outgoing fuel connection. Both connections are quick connect and are different diameters to prevent incorrect attachment of the fuel lines. All fuel passes through the fuel composition sensor before continuing on to the fuel rail. The fuel composition sensor measures two different fuel related parameters and sends an electrical signal to the powertrain control module (PCM) to indicate ethanol percentage, and fuel temperature.
The fuel composition sensor has a three-wire electrical harness connector. The three wires provide a ground circuit, a power source, and a signal output to the PCM. The power source is vehicle system voltage and the ground circuit connects to chassis ground. The signal circuit carries both the ethanol percentage and fuel temperature within the same signal.
The FCS uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temp, and change an output signal accordingly. The electrical characteristic of the FCS signal is a square-wave digital signal. The signal is both variable frequency and variable pulse width. The frequency of the signal indicates the ethanol percentage, and the pulse width indicates the fuel temperature. The PCM provides an internal pull-up to five volts on the signal circuit and the FCS pulls the 5 volts to ground in pulses. The output frequency is in direct proportion to the percentage of ethanol content in the fuel. The normal range of operating frequency is between 50 and 150 Hertz, with 50 Hertz representing 0 percent ethanol and 150 Hertz representing 100 percent ethanol. The normal pulse width range of the digital pulses is between 1 and 5 milliseconds, with 1 millisecond representing -40