Technical Description of OBD-II Trouble Code
The input for the Fuel Temperature Sensor B Circuit is too low.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies 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.
In my experience, a stored code P0187 indicates that the powertrain control module (PCM) has detected a voltage signal from the fuel temperature sensor (or circuit) that is lower than expected. The fuel temperature sensors I have dealt with are integrated into the fuel composition sensor. The “B” refers to an area of circuitry rather than a specific circuit or component.
The fuel composition sensor is designed to provide the PCM with an accurate analysis of fuel composition and temperature. It is typically a small computerized device located between the fuel tank and the fuel rail. As fuel passes through, the fuel composition sensor monitors the percentage of ethanol, water, and other contaminants. It provides input to the PCM in the form of square waveform voltage representations, which indicate the level of contamination in the fuel. A shorter waveform frequency represents a higher percentage of fuel contamination. The waveform frequency can also be described as the vertical posting of the pattern.
The percentage of ethanol in the fuel is usually categorized independently of other contaminants. Flex fuel vehicles can operate efficiently even with fuel composition levels as high as eighty-five-percent ethanol. Fuel temperature is calculated based on the pulse width or the horizontal portion of the waveform. A wider pulse width signal indicates a higher temperature of the fuel passing through the fuel composition sensor. Most systems use a pulse width that varies between one and five milliseconds.
If the fuel temperature is not within a specified range or is much lower than the ambient temperature, a P0187 code will be stored and a malfunction indicator lamp (MIL) may illuminate. Some vehicles may require multiple ignition cycles with a failure for the MIL to illuminate.
Code Severity & Symptoms
Due to the use of fuel temperature in calculating fuel delivery strategy, this code should be considered severe.
Symptoms of a P0187 code may include:
Causes
Potential causes for this code to be set are:
Diagnostic and Repair Procedures
A good starting point is to check for technical service bulletins (TSB) specific to your vehicle. There may be a known issue with a known fix provided by the manufacturer, which can save time and money during diagnosis.
In my opinion, the most comprehensive tool for diagnosing a code P0187 is a diagnostic scanner with an integrated digital volt-ohm meter (DVOM) and an oscilloscope. An infrared thermometer and a vehicle information source like All Data DIY can also be useful.
A visual inspection of all related wiring harnesses and connectors is an excellent starting point for any diagnosis. Damaged or burnt wiring and connectors should be repaired or replaced. After completing these repairs, the codes should be cleared and the system retested. From experience, I have found that even the OBD II readiness mode can be helpful when multiple failure cycles are required for MIL illumination. Simply complete the repairs, clear the codes, and drive the vehicle normally. If the PCM enters readiness mode, the repairs were successful. However, if the code is reset, it indicates that a malfunction still exists.
The fuel composition sensor is typically supplied with a five-volt reference signal and a ground. As a variable resistance sensor, it completes the circuit and provides the PCM with a fluctuating fuel temperature voltage signal. Next, I would test the reference voltage and ground at the fuel temperature sensor connector using the DVOM. If there is no reference voltage at the fuel temperature sensor connector, I would use the DVOM to test the corresponding circuits at the PCM connector. If no reference voltage is found at the PCM connector, it suggests a faulty PCM or a PCM programming error as the possible cause of the malfunction.
Using the oscilloscope, observe live data in waveform patterns by connecting the test leads to the ground and signal circuits, if both the reference and ground are present at the fuel temperature sensor connector. Compare the actual fuel temperature obtained using the infrared thermometer with the temperature reflected by the waveform patterns on the oscilloscope. If the fuel temperature reflected by the fuel temperature sensor does not coincide with the thermometer reading, it is likely that the fuel temperature sensor is defective.
Additional diagnostic notes:
