Technical Description of OBD-II Trouble Code
Intermittent Circuit Issue with Fuel Temperature Sensor B
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.
In simple terms, a stored code P0189 indicates that the powertrain control module (PCM) has detected an intermittent or erratic voltage signal from the fuel temperature sensor (or circuit). The “B” refers to a specific area of circuitry rather than a particular circuit or component.
In OBD II equipped vehicles, fuel temperature sensors are typically integrated into the fuel composition sensor housing. The fuel composition sensor provides the PCM with accurate analysis of fuel composition and temperature. This small computerized mechanism is located between the fuel tank and the fuel rail.
The fuel passing through the fuel composition sensor is evaluated to determine the percentage of ethanol, water, and other contaminants. Voltage patterns in the form of square waveforms are sent to the PCM based on the level of contamination in the fuel. Higher concentrations of fuel contamination are indicated by shorter waveform frequencies. The waveform frequency represents the vertical posting of the pattern.
Flex fuel vehicles can typically handle fuel composition levels as high as eighty-five-percent ethanol. The horizontal portion of the waveform represents fuel temperature and is recognized by the PCM as pulse width. The broader the pulse width of the waveform, the higher the temperature of the fuel passing through the fuel composition sensor. OBD II vehicles usually have a pulse width variation between one and five milliseconds.
If the fuel temperature signal appears intermittent or erratic, a P0189 code will be stored and a malfunction indicator lamp (MIL) may illuminate. Most OBD II equipped vehicles require multiple ignition cycles with a failure for the MIL to illuminate.
Code Severity & Symptoms
This code should be considered severe because fuel temperature is used to calculate fuel delivery strategy in flex fuel vehicles. Symptoms of a P0189 code may include:
Causes
Possible 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.
A diagnostic scanner with an integrated DVOM and an oscilloscope is the best tool for diagnosing a code P0189. A vehicle information source like All Data DIY and an infrared thermometer can also be very helpful.
Begin the diagnosis with a visual inspection of all related wiring harnesses and connectors. Repair or replace any damaged, burnt, or corroded wiring and connectors. Next, clear the codes and retest the system. Utilize the OBD II readiness mode when multiple failure cycles are required for MIL illumination. After completing the repairs, clear the codes and drive the vehicle normally. If the PCM enters readiness mode, the repairs were successful. If the code is reset, the malfunction still exists. The fuel composition sensor, which is of the variable resistance variety, receives a five-volt reference signal and a ground. Test the reference voltage and ground at the fuel temperature sensor connector using the DVOM. If no reference voltage is found, use the DVOM to test the corresponding circuits at the PCM connector. If no reference voltage is discovered at the PCM connector, suspect a defective PCM or a PCM programming error. Consider PCM failure only as a last resort.
Using the oscilloscope, monitor live data in waveform patterns if the reference and ground are both present at the fuel temperature sensor connector. Use the infrared thermometer to obtain the actual fuel temperature and compare it with the temperature indicated by the waveform patterns on the oscilloscope. If the fuel temperature revealed by the waveform voltage patterns does not match that of the thermometer, suspect a faulty fuel temperature sensor.
Additional diagnostic notes:
