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Technical Description of OBD-II Trouble Code
Range/Performance Issue with Fuel Temperature Sensor B Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to OBD-II equipped vehicles (Nissan, Ford, Fiat, Chevrolet, Toyota, Dodge, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When encountering code P0186, it indicates that the powertrain control module (PCM) has detected a voltage signal from the fuel temperature sensor (or circuit) that is outside the programmed range. The “B” refers to a specific area of circuitry rather than a particular circuit or component.
The fuel temperature sensor is typically integrated into the fuel composition sensor. This fuel composition sensor is a small computerized device placed in-line between the fuel tank and the fuel rail. Its purpose is to provide the PCM with an accurate reading of the fuel composition and temperature.
As fuel flows through the sensor, it is monitored to determine the presence of ethanol, water, and other contaminants. An electrical signal is sent to the PCM, indicating the type and degree of fuel contamination. The level of contamination is measured based on the percentage of contaminants in relation to the fuel and is input to the PCM as square waveforms. The frequency of the waveform reflects the percentage of fuel contamination, with a higher frequency indicating a higher percentage. The fuel composition sensor measures ethanol levels differently from other contaminants. Flex fuel vehicles can tolerate fuel composition levels as high as eighty-five-percent ethanol.
The pulse width, which represents the horizontal portion of the waveform, indicates the fuel temperature. A broader pulse width corresponds to a higher temperature of the fuel passing through the fuel composition sensor. On most models, pulse width modulation ranges from one to five milliseconds.
If the PCM detects an input signal from the fuel composition sensor that indicates the fuel temperature is outside the programmed range or varies significantly from the ambient temperature, it will store a P0186 code and may illuminate the malfunction indicator lamp (MIL). In some cases, multiple ignition cycles with a failure may be required for MIL illumination.
Code Severity & Symptoms
Since fuel temperature is crucial for calculating fuel delivery strategy in flex fuel vehicles, this code should be considered severe.
Symptoms of a P0186 code may include:
Causes
Possible causes for this code to be set are:
Diagnostic and Repair Procedures
It is always a good idea to check for technical service bulletins (TSB) specific to your vehicle as a starting point for diagnosis. This can save time and money by providing known issues and their fixes from the manufacturer.
To diagnose a code P0186, I typically use a diagnostic scanner, a digital volt/ohmmeter (DVOM), an oscilloscope, an infrared thermometer, and a vehicle information source like All Data DIY. A scanner with a built-in DVOM and portable oscilloscope would be useful.
A visual inspection of all related wiring harnesses and connectors is a good starting point for any diagnosis. Any damaged or burnt components should be repaired or replaced before retesting the system. The OBD II readiness mode can be helpful when multiple failure cycles are required for MIL illumination. After completing all repairs, I clear the codes and drive the vehicle as usual. If the PCM enters readiness mode, I know that my repairs were successful. If the code is reset, a malfunction still exists.
The fuel temperature sensor, integrated into the fuel composition sensor, is typically supplied with a five-volt reference voltage and a ground. The variable resistance sensor completes the circuit and provides the PCM with a fluctuating fuel temperature voltage signal.
The DVOM can be used to test the reference voltage and ground at the fuel temperature sensor connector. If reference voltage is not present, use the DVOM to test the corresponding circuits at the PCM connector. Repair any open circuits as necessary. If no reference voltage is found at the PCM connector, suspect a faulty PCM or a programming error.
If both the reference voltage and ground are present at the fuel temperature sensor connector, connect the oscilloscope’s test leads to the ground and signal circuits to observe live data in the form of waveform patterns. Use an infrared thermometer to measure the actual fuel temperature and compare it with the temperature reflected by the waveform patterns on the oscilloscope. If the fuel temperature indicated by the fuel temperature sensor does not match the thermometer reading, suspect a defective fuel temperature sensor.
Additional diagnostic notes:
Professional Service
While we always recommend seeking mechanical solutions for this issue, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
