P0189 Fuel Temperature Sensor B Circuit Malfunction

OBD-II DTC Datasheet

Fuel Temperature Sensor B Circuit Malfunction

What does this mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to OBD-II equipped vehicles (Nissan, Ford, Fiat, Chevrolet, Toyota, Dodge, etc.). Although general in nature, the specific repair steps may differ depending on the brand / model.

Simply put, a stored code P0189 means the powertrain control module (PCM) has detected a voltage signal from the fuel temperature sensor (or circuit) that is intermittent or erratic. “B” refers to the scope of a circuit, and not to a specific circuit or component.

Fuel temperature sensors in OBD II equipped vehicles are usually built into the body of the fuel composition sensor. The fuel composition sensor provides the PCM with an accurate analysis of the fuel composition and fuel temperature. A small computerized mechanism, the fuel composition sensor is located on the line 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. Square waveforms (representing voltage) are entered into the PCM according to the fuel contamination level. Higher concentrations of fuel contamination are indicated by shorter waveform frequencies. The frequency of the waveform can also be explained as the vertical traverse of the sample.

Flex-fuel vehicles generally work well with fuel formulations of up to eighty-five percent ethanol. The horizontal part of the waveform represents the temperature of the fuel and is recognized by the PCM as a pulse width. The wider the waveform pulse width, the higher the temperature of the fuel passing through the fuel composition sensor. One to five milliseconds (hundredths of a second) is a common change in pulse width in OBD II vehicles.

If the fuel temperature signal appears to be intermittent or intermittent, a P0189 code will be stored and a malfunction indicator lamp (MIL) may illuminate. Most OBD II equipped vehicles will require multiple ignition cycles (with failure) to illuminate the MIL.

Code severity and symptoms

This code should be considered severe because the fuel temperature is used to calculate the fuel delivery strategy in flex-fuel vehicles. Symptoms of a P0189 code may include:

Other fuel composition codes may be present.
Possible MIL illumination
There may be no obvious symptoms

reasons

Possible reasons for setting this code:

handle, shorted or damaged wiring or connectors
Defective temperature / fuel composition sensor
Intake air temperature sensor defective
Bad ambient temperature sensor
PCM or PCM programming error

Diagnostic and repair procedures

A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.

A diagnostic scanner with built-in DVOM and oscilloscope is arguably the best tool to diagnose the P0189 code. A vehicle information source (like All Data DIY) and an infrared thermometer can also be very helpful.

A good start to any diagnostic is to visually inspect all associated wiring harnesses and connectors. Damaged, burnt or corroded wires and connectors must be repaired or replaced. Then you need to clear the codes and retest the system. I like to use OBD II ready mode when it takes multiple fail cycles to illuminate the MIL. When the repair is complete, clear the codes and then drive normally. Your repair was successful if the PCM goes into readiness mode. If the code is cleared, the fault persists. The variable resistance fuel composition sensor is supplied with a reference voltage of XNUMX and ground. The fuel composition sensor closes the circuit and provides a fuel temperature swing voltage signal to the PCM. Check the reference voltage and ground at the fuel temperature sensor connector using the DVOM. If you cannot find a voltage reference at the fuel temperature sensor connector, use the DVOM to test the corresponding circuits at the PCM connector. If no voltage reference is found on the PCM connector, suspect a defective PCM or a PCM programming error. Consider PCM failure only as a last resort.

Using an oscilloscope, you can monitor real-time data in graph form if a reference signal and ground are present at the fuel temperature sensor connector. Use an infrared thermometer to get the actual fuel temperature and compare it with the temperature displayed on the oscilloscope charts. If the fuel temperature, as measured by the waveform voltage charts, does not match the thermometer temperature, suspect that the fuel temperature sensor is faulty.

Additional diagnostic notes:

Use the DVOM to test the resistance of the fuel temperature sensor according to the manufacturer's recommendations.
NOTE: Disconnect all related controllers before testing circuit resistance with the DVOM.

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