P0186: Fuel Temperature Sensor B Circuit Range/Performance

What Does The P0186 Code Mean?

When I come across a code P0186, I know that it means that the powertrain control module (PCM) has detected a voltage signal from the fuel temperature sensor (or circuit) that is not within a programmed range. “B” refers to an area of circuitry as opposed to a particular circuit or component.

The fuel temperature sensor is typically integrated into the fuel composition sensor. Designed to provide the PCM with an accurate composition and fuel temperature reading, the fuel composition sensor is a small, computerized device placed in-line between the fuel tank and the fuel rail.

As fuel passes through the sensor, it is monitored to determine how much ethanol, water, and other contaminants are present. An electrical signal to the PCM reflects not only what contaminants are present but also to what degree the fuel has been contaminated. The level of contamination is measured according to the percentage of contaminants (to fuel) and is input to the PCM as square waveforms.

The closer the waveform frequency, the higher the percentage of fuel contamination. Frequency could also be described as the vertical portion of the waveform. The fuel composition sensor measures the amount of ethanol present in the fuel differently than other contaminants. Fuel composition levels that are as high as eighty-five-percent ethanol are acceptable in flex fuel vehicles.

The pulse width, or the horizontal portion of the waveform, is indicative of fuel temperature. The broader the pulse width, the higher the temperature of the fuel passing through the fuel composition sensor. On most models, pulse width modulation varies between one and five-milliseconds (hundredths-of-a-second).

If the PCM detects an input signal from the fuel composition sensor, indicating that the fuel temperature is not within a programmed range or if the fuel temperature varies by a greater degree than expected from ambient temperature, a P0186 code will be stored and a malfunction indicator lamp (MIL) may be illuminated. Multiple ignition cycles (with a failure) may be required for MIL illumination in some applications.

What Are The Symptoms Of The P0186 Code?

Symptoms of a P0186 code may include:

• There may be no symptoms
• Other fuel composition codes may be present
• Possibly MIL illumination

What Are The Potential Causes Of The P0186 Code?

Potential causes for this code to set are:

• A bad ambient temperature sensor B
• Defective fuel temperature/composition sensor
• Faulty intake air temperature sensor
• Open, shorted, or damaged wiring or connectors
• PCM or a PCM programing error

How Serious Is This P0186 Code?

Fuel temperature is monitored by the PCM to calculate fuel delivery strategy (in flex fuel vehicles) so this code should be treated as severe.

How Can You Fix The P0186 Code?

Preparation

A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known issue with a known fix put out by the manufacturer and can save you time and money during diagnosis.

I would typically gain access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), an oscilloscope, an infrared thermometer, and a vehicle information source like All Data DIY to diagnose a code P0186. A scanner with a DVOM and a portable oscilloscope (built-in) would serve you well.

Step 1

A good place to start any diagnosis is with a visual inspection of all related wiring harnesses and connectors. All damaged or burnt components will need to be repaired or replaced; then retest the system. I have found that, when multiple failure cycles are required for MIL illumination, using the OBD II readiness mode can be helpful. When all repairs are completed, I clear the codes and drive the vehicle as usual. I know that my repairs were successful if the PCM enters readiness mode. I know that a malfunction still exists if the code is reset.

Step 2

Integrated into the fuel composition sensor, the fuel temperature 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 reference voltage and ground at the fuel temp sensor connector. Use the DVOM to test the corresponding circuits at the PCM connector if reference voltage is not present. Repair open circuits as required. If no reference voltage is discovered at the PCM connector, suspect a faulty PCM or a programming error.

If the reference and ground are both present at the fuel temperature sensor connector, connect the test leads of the oscilloscope to the ground and signal circuits to observe live data in the form of waveform patterns. Measure actual fuel temperature using the infrared thermometer and compare it with the temperature reflected by the waveform patterns on the oscilloscope.

Suspect that the fuel temperature sensor is defective if the fuel temperature reflected by the fuel temperature sensor fails to coincide with that of the thermometer.

Additional diagnostic notes:

• Use the DVOM to test fuel temperature sensor resistance according to manufacturer’s recommendations
• NOTE: Disconnect all related controllers prior to testing circuit resistance with the DVOM

Recommended Parts

Below are some recommended auto parts to help you address the trouble code affecting your vehicle and get it running smoothly again:

>>> Fuel Temperature Sensor
>>> MAT Sensor
>>> Ambient Air Temperature Sensor
>>> Twippo 372Pcs Waterproof Wire Connectors Kit
>>> INNOVA 5210
>>> MakerHawk LCD Handheld Digital Oscilloscope
>>> KAIWEETS Digital Multimeter

Note: During the purchasing process, please check carefully whether the part you want to buy fits your car!

Reference Sources

P0186: Fuel Temperature Sensor B Circuit Range/Performance, OBD-Codes.