Error Code P0179 is defined as Fuel Composition Sensor Circuit High Input. This means the PCM (powertrain control module, or also known as ECM or engine control module) has detected an abnormally high voltage from the fuel composition sensor (flex fuel sensor) or circuit. This particular sensor is designed to detect fuel contamination (particularly ethanol) and provide an accurate reading of the fuel temperature for the PCM.

This code is a generic trouble code, meaning, it appears to all vehicles equipped with OBD-II scanner, which are vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repair of course, vary from one make/model to another.

Definition

The PCM does a lot of jobs, one of which is to detect input signal from the fuel composition sensor which indicates the presence of non-gasoline (or non-ethanol in flex fuel vehicles) at a volume higher than 10% (For normal operation, a gasoline engine should not have more than 10% ethanol alcohol.

Some vehicles come with FF (Flexible Fuel) system which allows the engine to run up to 85% ethanol in its fuel. You can verify your vehicle’s manual for the exact specifications).  When this happens, there’s a fuel composition sensor that sends frequency modulated signals to the PCM, indicating a problem. The PCM will then trigger the error code P0179 and illuminate the Check Engine light. Some vehicles may require multiple failure cycles to illuminate the check engine light.

High circuit input can result from multiple problems, it can be a result of fuel contamination or the connection or the sensor, or melted wire causing a short in the sensor wires altogether.

Common Symptoms

Obviously, this code activates the Check Engine light, which is the most common tell-tale sign of the problem. For the symptoms, this error code may vary from non to severe engine misfire, depending on the nature of its cause. Some of the most common symptoms include:

  • Hesitation, sagging or stumbling on acceleration
  • No start condition
  • Excessive water in the fuel causing the engine problems if the sensor can’t detect it

Possible Causes

Fuel contamination is easily the most common cause of this error code. Other possible causes include:

  • Damaged, open, or shorted wiring connectors
  • Defective fuel composition sensor
  • Faulty PCM (rare)

How to Check

To diagnose, mechanics start their check up by checking the scan codes and documenting the freeze frame data. Clears the codes and then verify the codes to see if they return.

Next, they conduct a visual inspection of all the connections and wirings to the sensor to check for shorts in connections or wirings.

Then, they disconnect the connection to the sensor and check to see whether the sensor connection is contaminated or corroded, which could be shorted.

How to Fix

Fixing this error code is as easy as its diagnosis. Draining off the water in the fuel filter and clearing the codes to get rid of the Check Engine light is the first step.

Replace shorted or defective sensor

Repair or replace the short circuit, burnt wirings or connectors to the sensor

Replace fuel filter housing with the sensor and clearing the codes

Your engine will still start and run despite of excessive water in the fuel and sensor not detecting it. However, the symptoms such as misfiring, hesitation and stall will be your drivability issues.

When troubleshooting, conduct a visual inspection to the sensor. Make sure they’re making good contact. Connections can be shorted from not being properly serviced by the filter, or the pins could be bent on the connector or sensor.

If you disconnect the sensor connector and retest the system and the P0179 is gone, and had been replaced by codes P0178 or P0177, then that means the wiring is not shorted.