The fuel composition sensor measures the ratio of ethanol to gasoline in vehicles equipped with flexible fuel systems. It sends its readings to the powertrain control module (PCM), which tweaks parameters, such as fuel delivery and ignition timing, in response.
If the PCM detects the wrong electrical signal from the fuel composition sensor, it will set the engine code P0178.
The Diagnostic Trouble Code (DTC) P0178 indicates “Fuel Composition Sensor Circuit Low.”
Most vehicles can only burn fuels with a maximum of 10-15 percent ethanol alcohol before their engine starts developing issues. That’s because ethanol fuel has a higher octane rating and burns leaner than gasoline. A flexible fuel system lets the vehicle burn fuel with as much as 85 percent ethanol (E85).
Also called the “flex fuel sensor,” the fuel composition sensor measures the percentage of ethanol in the fuel. It sends the appropriate frequency modulated signal to the PCM. Lower voltages indicate low ethanol levels while higher voltages show that the ratio is skewed toward ethanol.
The readings from the fuel composition sensor tell the PCM how to adjust parameters like the amount of fuel sprayed into the engine’s cylinders and the timing of the spark plugs’ ignition. This makes the sensor and its electrical circuit invaluable to making sure the engine gets the right amount of ethanol fuel.
The PCM regularly checks the signals from the fuel composition sensor. If the signal voltage is lower than what it should be, the computer will set the P0178 code “Fuel Composition Sensor Circuit Low.” This typically means the frequency of the sensor is below a certain range.
GM describes the operation of the fuel composition sensor this way:
The PCM provides an internal pull up to 5 volts on the signal circuit, and the FCS pulls the 5 volts to ground in pulses. The normal range of operating frequency is between 50–150 Hz. The normal pulse width ranges between 1–5 milliseconds. If the PCM detects a signal less than 45 Hz DTC P0178 will set.
You can find more information that can help you better understand codes like P0178 in our technical discussion about the fuel composition sensor.
Note: The definition of code P0178 may differ according to the vehicle manufacturer. Check the appropriate repair manual or repair database for the exact code definition.
The DTC P0178 is a generic engine code. It applies to many OBD-compatible vehicles that can burn flex fuel.
If you’re not confident about conducting diagnostic tests on your vehicle’s fuel position sensor circuit, it’s a good idea to bring it to a professional mechanic. Otherwise, you may go ahead and diagnose the issue yourself.
Check out this video to learn more about what the diagnostic procedure might entail:
Trying to repair a P0178 code may become a frustrating task if you lack the right tools and knowledge to test the faulty fuel composition sensor, PCM, and wiring. In most cases, it’s best to leave the job to professionals.
For DIYers with advanced technical knowledge and hands-on experience who prefer to test and replace their vehicle’s fuel composition sensor on their own, it will be helpful to rely on the appropriate repair manual or refer to an online repair database.
Not all flex fuel vehicles use a fuel composition sensor. Some use algorithms in the engine controller and O2 sensor input to determine fueling strategy.
But for just one example, on GM vehicles equipped with fuel composition sensors (FCS), the fuel flows through the sensor, which produces a Hertz (cycles per second) signal the ECM/PCM uses to determine alcohol content. The FCS is a three-wire sensor with an internal microprocessor.
Not all flex fuel vehicles use a fuel composition sensor. Some use algorithms in the engine controller and O2 sensor input to determine fueling strategy.
– Richard McCuistian, ASE Certified Master Automobile Technician
The FCS uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temperature, and changes output signals accordingly. The signal circuit carries both the ethanol percentage, and the fuel temperature within the same signal. The signal is both variable frequency and variable pulse width. The frequency signal indicates the ethanol percentage and the pulse width indicates fuel temperature.
If the FCS over-reports the alcohol content, it can cause hard starting concerns even if no diagnostic trouble code is stored, because the ECM/PCM has no way of knowing whether the FCS is telling the truth or not if the failure is what is known as an “in range” failure.” And there are no rationality checks to verify alcohol content.
Where an in-range failure is concerned, if the ECM/PCM believes the alcohol content of the fuel is, let’s say, 50% when the fuel is actually only 10% alcohol, the ECM’s algorithms will over-fuel the engine during cold starting and foul the spark plugs. This hard start/no-start concern may be intermittent. It may start sometimes and not other times. If the scan tool is reporting fuel alcohol levels that are known to be much higher than the known alcohol content of fuel, this may be the case.
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