P0186 OBD-II Trouble Code: An Out of Range Performance for Fuel Temperature “B” Circuit

OBD Codes

By Rodney (Mechanics)

The Definition of Trouble Code P0186

An out-of-range performance for fuel temperature sensor “B” circuit.
If you ever come across the trouble code P0186 on your vehicles, then you should know that it’s probably related to a problem with the unusual voltage signal on the fuel temperature sensor that is detected by the PCM. The voltage is supposedly to be out of the programmed performance range. The “B” in this case means the specific area of the circuit instead of the whole sensor component.

The fuel temperature sensor inside your vehicle is usually built together with the fuel composition sensor. They are designed to read and analyze the composition and temperature of fuel inside your tank and will send the reading to the PCM. You can usually see these components as some small computerized elements in the engine, located between the fuel tank and rail.

The sensor works by reading the flow of fuel that goes through it. It will then analyze the level of contaminants, such as ethanol or water, inside the fuel combination. The PCM will receive electrical signals about the level of those contaminants currently present in the fuel. The signal is sent in the form of square waveforms where the PCM will analyze the data. If the frequency from the waveform is lower than normal, then the PCM will notify you that the level of contaminants is currently high in the fuel. But in a special case for ethanol, the PCM will allow for up to 85% of this element in the fuel as an acceptable level.

OBD II Code Fuel Temperature Sensor Circuit

The PCM will also analyze the pulse width from the waveform to determine the current fuel temperature. If the pulse width is broader than usual, then the fuel temperature is higher than normal. The variation for pulse width on most vehicles is usually between 1 – 5 milliseconds.

When the PCM is detecting a reading from the fuel composition sensor that says the fuel temperature is spiking compared to normal, or the fuel composition is not within the range of performance for the engine, then it will store trouble code P0186 in its memory. Also, the malfunction indicator lamp (MIL) will be turned on to notify you about the problem. In some cases, the MIL light will require a few failure cycles before it gets turned on.

What Does Trouble Code P0186 Mean?

Trouble code P0186 is an OBD-II fault code that will be stored by the PCM when there is an out-of-range fuel composition and temperature reading from the fuel temperature sensor B. It might also indicate that the voltage on the sensor circuit has gone more or less than the acceptable specifications from the manufacturer.

The Possible Causes for Trouble Code P0186

  • Your engine control module is detecting an out-of-range performance from the fuel composition sensor when the vehicle is started or driven.
  • An unstable condition of wiring and connections.
  • The ECM is experiencing short or open circuits.
  • Extreme outside weather conditions cause the fuel temperature to drop below -40 degrees Fahrenheit.
  • There are damages on the sensor connector that causes the fuel tank or fuel rail to act intermittently.
  • A defective condition on the ECM or shorted connection of the fuel composition sensor.

The Symptoms of Trouble Code P0186

  • The engine control module stores the trouble code P0186 to the memory along with turning on the Check Engine Light.
  • The fuel temperature has dropped below freezing point or there are contaminants in the fuel composition. Those conditions lead to the failure of engine ignition.
  • The fuel temperature becomes too hot which will lead to the engine stalling.
  • The excess level of contaminants drives the sensor to report erratic readings, which will lead to incorrect temperature for the fuel.

How Your Mechanics Should Check Trouble Code P0186

  • First, scan the trouble code and store the freeze frame data prior to the retesting phase after clearing the code.
  • Inspect the wiring and the connections related to the sensor to see if they’re experiencing any damaged conditions.
  • Test the condition of the sensor to see if it can perform normally after disconnecting the component.
  • Measure the fuel temperature manually and compare the reading with the sensor reading.
  • Inspect the working condition of the fuel heater to see that it is not causing the fuel temperature to get too high.

Common Mistakes When Trying to Diagnose Trouble Code P0186

  • Skipping the necessary visual inspection to see the condition of connections. This is important to make sure that there is a good connection between the fuel temperature sensor and the fuel heater.
  • Skipping the inspection for fuel contaminants that might cause the malfunctioning of the sensors.
  • Skipping the retest step to see if the fuel temperature can get higher in warmer areas. The cold outside temperature that is below the freezing point can cause the trouble code to appear.

Additional Diagnostic Notes

  • You can also check the resistance of the fuel temperature sensor and see if they work according to the specifications.
  • NOTE: Make sure that you disconnect all the related controllers before using the DVOM for the inspection process.

How Serious is Trouble Code P0186?

  • Your vehicle will have a hard time starting when there is too much water in the fuel composition. This condition can also be caused by the freezing fuel temperature or the damaged condition of the fuel heater.
  • The engine in the vehicle might experience misfiring or stalling when the fuel temperature is outside the normal range. This can be caused by the erratic performance of the fuel temperature sensors.

The Repair Steps for Trouble Code P0186

  • Inspect the condition of the fuel composition sensor and replace the component if necessary.
  • Replace the broken or defective wiring or connections that go to the sensors if they are open or shorted.
  • Replace the ECM if they are broken or defective.
  • Replace both the fuel heater and the temperature sensors.

Using the recommendations from the technical service bulletins is also a good idea to start the repairing process. The bulletins might have listed your issue as a known problem, and you can take the necessary steps required to fix the problem without spending money on unnecessary replacements.

The help from a DVOM that features an oscilloscope and infrared thermometer is recommended for the repair steps. One tool that you can use to diagnose the trouble code P0186 is the All Data DIY. This will help you to inspect the condition of the sensors along with the connections that go through them.

It might be beneficial for you if you start the inspection by checking the condition of the wiring harnesses along with other related connections to the sensors. Any damages on those components should be replaced with a new one promptly before retesting the whole system. Using the OBD-II readiness mode can also be helpful if the failure cycles happen multiple times. You can try clearing the trouble code and drive the vehicle to see if the code persists. If you notice that the PCM can enter the readiness more, then it means that the repair steps are successful. But, the code reappears, then the malfunction is not yet fixed.

Another part of the fuel composition sensor is the fuel temperature sensor. This sensor usually has a 5V reference voltage and ground. The PCM will receive the temperature reading from the variable resistance sensor in a form of fluctuating voltage signal for the temperature reading.

You can also use the DVOM to inspect the reference voltage and ground by connecting the respective circuits to the PCM connector. You can then fix any open or shorted circuit that is detected by the DVOM. If the tool cannot detect any reference voltage, then you might have a faulty PCM condition or a programming error on the component.

You can also use the oscilloscope to observe the waveform patterns from the fuel temperature sensors. Alternatively, the infrared thermometer can be used to measure the fuel temperature in the tank. You can then compare your readings with the number from the sensor. If the readings are different, then it might mean that the fuel temperature sensor is defective.

Additional Comments Regarding the Trouble Code P0186

Trouble code P0186 might appear on vehicles with diesel or gasoline engines that are designed with a fuel heater or fuel temperature sensors. If you have problems with those components, always consult the manufacturer’s recommended repair steps to avoid more problems when fixing them. Some specific sensor models might require special repair steps.