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P0178 – What Does It Mean and How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


P0178 is an OBD-II Code that refers to Fuel Composition Sensor Circuit Low Input


A P0178 code is a generic diagnostic trouble code that occurs when the PCM/ECM receives a signal indicating an issue with the fuel flex system. The fuel composition sensor’s readings are used by the PCM to change settings such as the quantity of gas sprayed into the engine’s cylinders and the spark plug ignition timing. The sensor and the electrical circuitry that controls it are critical to ensuring that the engine receives the correct quantity of ethanol fuel.

The PCM maintains a thorough eye on the signals coming from the fuel composition sensor. If the signal voltage is lower than it should be, the computer will display P0178 “Fuel Composition Sensor Circuit Low.”


Common causes for this code include:

  • Flexible Fuel (FF) sensor to 
  • Vehicle Power (VPWR) circuit harness open or shorted FF to VPWR circuit poor electrical connection 
  • Open in FF sensor signal circuit 
  • Short to ground in FF sensor signal circuit 
  • Fuel contamination Short to VPWR in FF sensor


The symptoms of a P0178 code are:

  • Illuminated Check Engine Light 
  • Reduced vehicle performance
  • There is no start or rough start condition
  • Increased gasoline usage


To diagnose a P0178 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure.
  2. Visually inspect the wiring harness and connectors for the FF sensor, looking for damaged, loose, or corroded terminals.
  3. Check the VPWR supply voltage to the FF sensor at the ECM connector.
  4. With a DVOM (digital volt ohm meter), test the FF sensor circuit between the PCM connector and ground for an open or short to ground.
  5. Test the resistance of the FF sensor.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P0178 code:

  • Not thoroughly inspecting all the wiring and connectors.
  • Not checking the VPWR supply voltage.
  • Not checking the resistance of the FF sensor.
  • Testing the FF sensor circuit for an open when it is actually a short to ground.
  • Failing to clear the codes and test drive the vehicle to see if the P0178 code returns.

How serious is this?

A P0178 code is considered a low to mid tier code. It indicates a problem with the fuel composition sensor or its circuitry, which can affect engine performance and gas mileage.

What repairs can fix the code?

repair manuals

The following are solutions that may fix this problem:

  • Draining off water in fuel filter and clearing Check Engine Light codes
  • Replacing the open defective sensor
  • Repairing the open circuit or burned wiring or connection to the sensor
  • Replacing the fuel filter housing with the sensor and clearing the codes

Related codes

A P0178 is related to and may be accompanied by the following codes:

P0179 – Fuel Composition Sensor Circuit High Input

P0231 – Fuel Pump Secondary Circuit Low Voltage

P0232 – Fuel Pump Secondary Circuit High Voltage

P0597 – Thermostat Heater Control Circuit Open or Shorted to Ground

P06DD – Fuel Composition Sensor “B” Circuit Range/Performance


In conclusion, the P0178 code is a diagnostic trouble code that refers to a problem with the fuel composition sensor or its circuitry. This code can affect engine performance and gas mileage. The most common repairs are draining off water in fuel filter, replacing the open defective sensor, repairing the open circuit or burned wiring or connection to the sensor, and replacing the fuel filter housing with the sensor.