Technical Description of OBD-II Trouble Code
The input for the fuel temperature sensor circuit B is too high.
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.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
I have discovered that a stored code P0188 indicates that the powertrain control module (PCM) has detected a higher-than-expected voltage signal from the fuel temperature sensor (or circuit). In my experience, fuel temperature sensors are usually integrated into the fuel composition sensor. The “B” designation refers to a specific area of circuitry rather than a particular circuit or component.
The fuel composition sensor provides the PCM with accurate analysis of the fuel composition and temperature. This computerized sensor is located between the fuel tank and the fuel rail.
As fuel flows through the fuel composition sensor, it assesses the percentage of ethanol, water, and other contaminants present. The PCM receives square waveform voltage patterns based on the level of fuel contamination. Shorter waveform frequencies indicate a higher percentage of fuel contamination. The waveform frequency can also be represented as the vertical position of the pattern. The amount of ethanol in the fuel is typically categorized separately from other contaminants. Flex fuel vehicles can operate normally even with fuel composition levels as high as eighty-five percent ethanol.
The PCM recognizes fuel temperature as the pulse width or the horizontal portion of the waveform. A thicker pulse width signal indicates a higher temperature of the fuel passing through the fuel composition sensor. Most vehicles use pulse width variations of one to five milliseconds.
If the fuel temperature appears significantly higher than the ambient temperature, a P0188 code will be stored and the 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
Due to the fact that fuel temperature is used to calculate fuel delivery strategy in flex fuel vehicles, this code should be considered severe. Symptoms of a P0188 code may include:
Causes
Potential causes for this code to be set are:
Diagnostic and Repair Procedures
A good starting point is always 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.
An effective tool for diagnosing a code P0188 is a diagnostic scanner with an integrated digital volt-ohm meter (DVOM) and oscilloscope. An infrared thermometer and a vehicle information source like All Data DIY can also be very helpful.
A visual inspection of all related wiring harnesses and connectors is a suitable first step in the diagnosis. Damaged, burnt, or corroded wiring and connectors will need to be repaired or replaced. Next, clear the codes and retest the system. Use the OBD-II readiness mode when multiple failure cycles are required for MIL illumination. After completing the repairs and clearing the codes, drive the vehicle normally. If the PCM enters readiness mode, the repairs were successful. If the code is reset, the malfunction still exists.
Fuel composition sensor operation typically involves a five-volt reference signal and a ground. The variable resistance fuel composition sensor completes the circuit and provides the PCM with a fluctuating fuel temperature voltage signal. Continue by testing 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, use the DVOM to test parallel circuits at the PCM connector. If no reference voltage is found at the PCM connector, a faulty PCM or PCM programming error may be the issue. Consider PCM failure only as a last resort.
Using the oscilloscope, you can observe live data in waveform patterns if the reference and ground are both present at the fuel temperature sensor connector. Connect the test leads to the ground and signal circuits and observe the waveform patterns on the display screen. Use the infrared thermometer to measure the actual fuel temperature and compare it with the temperature indicated by the waveform patterns on the oscilloscope. If the fuel temperature indicated by the waveform voltage patterns does not match that of the thermometer, suspect that the fuel temperature sensor is defective.
Additional diagnostic notes:
