Fuel Composition Sensor Circuit Range/Performance


The PCM has detected an abnormal voltage reading from the fuel composition sensor (flex fuel sensor) or circuit. This sensor is designed to detect fuel (ethanol) contamination and provide the PCM with an accurate fuel temperature.

Code Set Parameters

If the PCM detects an input signal from the fuel composition sensor that indicates the presence of non-gasoline (or non-ethanol in flex fuel vehicles) at a volume of greater than ten-percent (verify specifications for your particular vehicle), a code will be stored and a service engine soon lamp may be illuminated. Some vehicles require multiple failure cycles in order to illuminate the malfunction indicator lamp.


These may vary from none at all to a severe engine misfire, hesitation, sag, or stumble on acceleration, or a no start condition. Other fuel system codes may also be stored in addition to this code.

Common Causes

Fuel contamination is by far the most common cause of this code setting condition. Other causes may include a defective fuel consumption sensor, open, shorted, or damaged wiring or connectors, or a faulty PCM.

Common Misdiagnosis

Technicians report that the fuel composition sensor is frequently replaced in error when fuel contamination is at the root of the malfunction.


  • A scanner (or code reader - preferably a scanner), a digital volt ohmmeter, and an oscilloscope will be virtually indispensible for a successful diagnosis of this code setting condition. The fuel composition sensor, or flex fuel sensor as it is called in service part terms, is actually an inline microprocessor that effectively measures the percentage of gasoline (as compared to other liquids) that compose the fuel which passes through it en route to the fuel rail
  • The fuel composition sensor is also responsible for measuring the fuel temperature and transmitting this information to the PCM
  • The fuel composition sensor is a small computer that measures all of the fuel which passes through it to ensure that ethanol, water, and other contaminants do not exceed a certain percentage of the overall fuel makeup
  • It makes its own calculations concerning fuel composition and temperature and sends an input signal to the PCM
  • The electrical signal of the fuel composition sensor is seen by the PCM as a square wave form
  • The signal varies in frequency according to the degree of contaminants present in the fuel being pumped from the tank to the fuel rail
  • The pulse width of the square wave form pattern is received by the PCM as fuel temperature
  • The fuel composition sensor is supplied with a voltage reference (normally 5-volts) by the PCM
  • It also typically has a ground wire and a signal wire
  • The fuel composition sensor provides ground pulses in direct correlation to the amount of contaminants that are present
  • The typical range of frequency is from 50 to 150 Hertz; with 50 Hertz representing clean fuel and 150 Hertz representing 100-percent ethanol (or other contaminants)
  • The higher the degree of fuel contamination present, the greater the frequency of the voltage wave form frequency
  • Additionally, fuel temperature is measured by the fuel composition sensor in signal voltage pulse width
  • The digital pulse width range on most applications is between 1 and 5 milliseconds
  • Fuel that is at -40-degrees (F) should pulse at increments of 1-millisecond and fuel that is measured at 257-degrees (F) should provide a pulse width of 5-milliseconds
  • So, the higher the fuel temperature the greater the speed of the pulse width modulation to the PCM
  • If the sensor detects a voltage reading that is not within specs, according to the manufacturer, a code will be stored and a malfunction indicator lamp may be illuminated. Flex fuel vehicles can be operated with a much as 85-percent ethanol as fuel
  • These types of vehicles are programmed to adjust engine timing and fuel delivery strategy differently than conventional engines
  • In this type of system, the degree of ethanol present in the fuel is measured as a separate component from other contaminants
  • Begin your diagnosis by visually inspecting all wiring and connectors
  • Look for shorted or burned wiring and replace circuitry and connectors as required
  • If the system wiring, connectors, and components appear to be in normal working order, connect the scanner to the diagnostic connector and record all stored trouble codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. If the code immediately returns or the drivability condition persists, gain access to the fuel composition/fuel temperature sensor (usually on top of the fuel storage tank) and use the digital volt ohmmeter to check for reference voltage and ground signals at the sensor connector
  • If there is no voltage present, disconnect the electrical connector from the PCM and use the digital volt/ohmmeter to check for continuity and resistance on the reference circuit from the PCM
  • Repair or replace open or shorted wiring and/or connectors as required and retest the system
  • If no ground signal is present, obtain a wiring schematic from the manufacturer and perform a continuity and resistance test on all ground circuits
  • Compare your findings with manufacturer’s specifications and repair or replace ground circuits as required. If there is a reference voltage and ground signal present, connect the positive oscilloscope lead to the signal wire of the fuel composition sensor and the ground lead to a suitable vehicle ground
  • Observe wave form patterns and compare your findings with the manufacturer’s recommended specs and draw your conclusions from the live data
  • Compare actual fuel composition with sensor waveform frequency and actual fuel temperature data with fuel composition sensor waveform pulse width data
  • If sensor data fails to coincide with actual conditions, replace the sensor and retest the system to ensure a successful repair. If the waveform pulse width and frequency data coincides with actual conditions, suspect a defective PCM
  • Keep in mind that PCM failure is rare and replacement will require reprogramming