AutoCodes.com | All About OBDII Engine Codes
AutoCodes - GMC - P0178 GMC

P0178 GMC Code - Fuel Composition Sensor Circuit Low Frequency

- Code Popularity: 10,369 Views
- Repair Importance Level: 3/3
- Repair Difficulty Level: 3/3
Table of contents
Share on Facebook   Share on Twitter   Share on Linkedin

What are the Possible Causes of the DTC P0178 GMC?

NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
  • Faulty Fuel Composition Sensor (FCS) or Flex Fuel Sensor
  • Fuel Composition Sensor harness is open or shorted
  • Fuel Composition Sensor circuit poor electrical connection
  • Faulty Engine Control Module (ECM)
⚠ Does your GMC have Safety Recalls? → Check here

How to Fix the DTC P0178 GMC?

Review the 'Possible Causes' mentioned above and visually examine the corresponding wiring harness and connectors. Ensure to check for any damaged components and inspect the connector pins for signs of being broken, bent, pushed out, or corroded.

Code Tech Notes

Replacing the Fuel Composition Sensor (FCS) or Flex Fuel Sensor usually takes care of the code.





User Submitted Video

Search for P0178 GMC Questions and Answers: Search here →

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0178 GMC code, it typically requires 1.0 hour of labor. The specific diagnosis time and labor rates at auto repair shops can differ based on factors such as the location, make and model of the vehicle, and even the engine type. It is common for most auto repair shops to charge between $75 and $150 per hour.

What are Code Possible Symptoms?

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

Code P0178 GMC 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. 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.
Alcohol content information is supplied to the Engine Control Module (ECM) from the Fuel Composition Sensor. The Fuel Composition Sensor has an ignition positive circuit, a signal circuit, and a ground circuit. The Fuel Composition Sensor uses a microprocessor inside the sensor to measure the ethanol percentage and changes the output signal accordingly. The signal circuit carries the ethanol percentage via the frequency signal. The ECM provides an internal pull up to 5V on the signal circuit, and the fuel composition sensor pulls the 5V to ground in pulses. The normal range of operating frequency is between 50 - 150 Hz. The microprocessor inside the sensor is capable of a certain amount of self-diagnosis. An output frequency between 180 Hz and 190 Hz indicates that the fuel is contaminated. The Diagnostic Trouble Code (DTC) will be set when the ECM detects that the fuel composition sensor output frequency is less than 45 Hz for greater than 6s.

OBDII Code P0178 GMC - Fuel Composition Sensor Circuit Low Frequency - AutoCodes.com
P0178 GMC Code - Fuel Composition Sensor Circuit Low Frequency

Need More Information on the Code?

Need more information on how to fix the P0178 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P0178 GMC code diagnosed by a professional: Find a repair shop in your area

Code P0178 GMC comments

Help us improve AutoCodes.com. Leave a comment below or tell us if the information above helps you fix the code. What are the symptoms on your vehicle for the P0178 GMC code? Have you replaced any parts?
Any information is appreciated. Thanks