AutoCodes - Gmc - P0178 GMC

P0178 GMC Code - Fuel Composition Sensor Circuit Low Frequency

Repair Importance Level: 3/3
Repair Difficulty Level: 3/3
Table of contents

|P0178 GMC code possible causes

  • Faulty fuel composition sensor
  • Fuel Composition Sensor harness is open or shorted
  • Fuel Composition Sensor circuit poor electrical connection

|How do I fix the P0178 GMC code?

Check the "Possible Causes" listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector's pins.

|P0178 GMC code tech notes

Replaced Fuel Composition Sensor

|Cost of diagnosing the P0178 GMC code

Labor: 1.0

The cost of diagnosing the P0178 GMC code is 1.0 hour of labor. The auto repair's diagnosis time and labor rates vary by location, vehicle's make and model, and even your engine type. Most auto repair shops charge between $75 and $150 per hour.

|When is the P0178 GMC code detected?

The ECM detects that the fuel composition sensor output frequency is less than 45 Hz for greater than 6 s

|What are P0178 GMC code possible symptoms?

  • Engine Light ON (or Service Engine Soon Warning Light)

|P0178 GMC code description

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°C (-40°F), and 5 milliseconds representing 125°C (257°F).
The microprocessor inside the sensor is capable of a certain amount of self-diagnosis. An output frequency of 170 Hertz indicates either that the fuel is contaminated, contains methanol, or that an internal sensor electrical fault has been detected. Certain substances dissolved in the fuel can cause the fuel to be contaminated, raising the output frequency to be higher than the actual ethanol percentage should indicate. Examples of these substances include water, sodium chloride (salt), and methanol.
The FCS will indicate a slightly lower ethanol percentage than what is advertised at the fueling station. This is not a fault of the sensor. Government regulations require that alcohol intended for use as engine fuel be DENATURED. 100 percent pure ethanol is denatured with approximately 4½ percent gasoline before being mixed with anything else. When an ethanol gasoline mixture is advertised as E85, the 85 percent ethanol was denatured before being blended with gasoline. An advertised E85 fuel contains only about 81 percent ethanol. The FCS measures the actual percentage of ethanol in the fuel.
OBDII Code P0178 Gmc - Fuel Composition Sensor Circuit Low Frequency - AutoCodes.com

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