P0168 – Fuel temperature too high

Reinier

By Reinier (Contact Me)
Last Updated 2017-03-08
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P0168 Fuel temperature too high Wiring, fuel temperature sensor, mechanical fault

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What Does Code P0168 Mean?

SPECIAL NOTES: Diagnosing code P0168 accurately requires the use of an infrared thermometer and an oscilloscope, as well as access to reference data in the form of the relevant waveform library. While it may be possible to diagnose and repair this code without the required equipment in some cases, the process is largely governed by chance, and an outcome that is successful over the long term can therefore not be guaranteed.

It should also be noted that the setting parameters for this code vary greatly between applications on the one hand, and that bad or excessively contaminated fuel can play a major role in setting this code, on the other. For these reasons, the information provided here is intended for general informational purposes only, and it should therefore NOT be used in any diagnostic/repair procedure for this code without making proper reference to the manual for the application being worked on.

However, the generic diagnostic/repair information provided here should enable most non-professional mechanics to successfully diagnose and repair code P0168 on most applications, provided that apart from a repair manual for the application and a good quality digital multimeter, at least an infrared or laser-based thermometer is available. END OF SPECIAL NOTES. 

OBD II fault code P0168 is a generic code that is defined as “Fuel Temperature – Too High”, and is set when the PCM (Powertrain Control Module) detects an abnormally high fuel temperature under pre-set conditions for a period time set by the manufacturer. Note that on flex-fuel applications in particular, fuel temperature is a critical factor in the PCM’s fuel delivery calculations, meaning that code P0168 should NOT be ignored, or left unresolved for longer than is absolutely necessary.

On most applications, the fuel temperature sensor is incorporated into the fuel composition sensor, which, as the name suggests, has the function of analyzing the composition of the fuel that passes through it. This is required because of the variability of many fuels sold today; in many cases, fuel that should contain only say, 15% ethanol, can contain as much as 25% or more, or as little as 10% or less.

Since the composition of fuel determines the calorific (energy) value of that fuel, this is important, because the PCM needs to “know” how much energy a given volume of fuel contains in order to calculate a fuel delivery strategy that is appropriate for any given set of operating conditions. Moreover, the fuel composition sensor also measures the amount of water, or other non-fuel contaminants in the fuel, which contaminants can have a huge influence on the temperature of the fuel that is ultimately delivered to the injectors.

In practice, the fuel composition sensor generates a square waveform, in which the frequency, or pulse width, of the waveform relates to the temperature of the fuel. Put in another way, the hotter the fuel is, the closer the horizontal parts of the waveform follow each other. Typically, the frequency of these waveforms varies between one and five milliseconds.

Thus, the signal that is sent to the PCM by the combined fuel composition/temperature sensor contains information on both the composition and temperature of the fuel. However, since the average home-based mechanic does not have the means (for instance, access to a laboratory) with which to double check the actual composition of the fuel, checking the actual fuel temperature with an accurate thermometer is the only cost effective way to obtain a working correlation between the composition and the temperature of the fuel in the system, hence the need for an oscilloscope and a thermometer.

From the above it should be obvious that factors other than the actual fuel temperature can cause this code to set. Regardless of the actual cause though, the PCM will set this code and illuminate a warning light when it receives a signal that indicates an excessively high fuel temperature. Note that on some applications, several failure cycles may be required before the PCM will set code P0168, or illuminate a warning light.

The image below shows an approximation of what typical temperature-related waveforms might look like for cold and hot fuel. In this image, the wide separations between each successive wave in the top sequence indicate cold fuel, and the smaller divisions (higher frequency) in the bottom sequence indicate a higher fuel temperature.

Bear in mind though that the image below is intended for illustrative purposes only, and the actual waveform shown on the oscilloscope must be compared to the reference data for the application to obtain accurate test results.

Square Waveforms

What are the common causes of code P0168 ?

Some common causes of code P0168 could include the following-

  • Damaged, shorted, disconnected, burnt, or corroded wiring and/or connectors
  • Defective fuel composition/temperature sensor
  • Excessively contaminated fuel
  • Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.

What are the symptoms of code P0168 ?

Common symptoms of code P0168 could include the following-

  • Stored trouble code and an illuminated warning light
  • Other fuel composition-related codes could be present
  • Codes indicting short circuits in the fuel system in general, and in the fuel composition / temperature sensor and/or circuit may be present
  • Note that in some cases, code P0168 may be present as a pending code for multiple drive cycles before becoming an active code.

NOTE: Code P0168 very rarely, if ever, produces noticeable driveability-related symptoms.

How do you troubleshoot code P0168 ?

SPECIAL NOTES: Non-professional mechanics are strongly urged to read the section in the manual that describes the fuel composition/temperature sensor before attempting a diagnosis and/or repair of this code. Gaining at least a working knowledge and understanding of the function of this sensor, as well as its control circuit could prevent the unnecessary replacement of parts and components since in some cases, replacing the fuel in the application with high quality branded fuel could resolve the issue. END OF SPECIAL NOTES.

 

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

NOTE: Take careful note of other codes that may be present along with P0168, and particularly codes that indicate short circuits or high resistances in the fuel system. Refer to the manual for detailed information on the possible relationship between additional codes and P0168, and repair these codes first if the manual recommends it, since these types of codes are often the direct cause of P0168.

Step 2

Since poor fuel quality is often implicated in the setting of this code, consider the possibility that the fuel in the vehicle might be the cause of the problem. Thus, if the code appeared soon after adding fuel, consider replacing that fuel, or adding high quality branded fuel in an effort to resolve the code.

At this point, it might be a good idea to take a reading of the fuel’s actual temperature if an oscilloscope and reference data is not available. Compare this reading with the value stated in the manual, since there is no single fuel temperature that is valid for all applications under all conditions. Most manufacturers specify exact temperatures the fuel must be at under a variety of conditions, so make sure to obtain as many temperature readings as the manual demands in order to obtain the most accurate test results.

If the actual fuel temperature is available, it becomes easier to determine whether a reading from the fuel composition/temperature sensor is plausible or not. However, making this determination with the aid of an oscilloscope is much easier, since all guesswork is removed from the equation, so to speak.

Step 3

If the code persists, but the fuel in the vehicle is known to be good, and there are no additional codes present (including pending codes), refer to the manual to locate and identify all relevant components and wiring. Also, use this opportunity to determine the location, routing, function, and color-coding for all associated wiring.

Perform a thorough visual inspection of all associated wiring, and look for damaged, burnt, shorted, disconnected, or corroded wiring and connectors. Make repairs as required, clear all codes, and operate the vehicle for at least one complete drive cycle to see if the code returns.

Step 4

If the code persists but the wiring is free of visible damage, prepare to perform reference voltage, ground, resistance, and continuity checks on all associated wiring. However, be sure to disconnect all wiring from the PCM and other affected controllers during resistance and continuity checks to prevent damage to one or more controllers.

Pay particular attention to the 5-volt reference and ground circuits. In most cases, the ground is supplied by the PCM, so if obtained readings do not conform to specified values, test the relevant pins on the PCM for ground and a 5-volt current in the reference voltage circuit. Replace the PCM if it delivers neither the ground, nor the reference voltage. Keep in mind though that PCM failure is rare, and the fault is therefore much more likely to be the result of some other failure, such a failure of the fuel composition / temperature sensor itself.

Bear in mind also that the sensor forms part of the circuit, and as such, its internal resistance and continuity must be tested as well. Consult the manual on the correct procedure to follow to test the sensor, and replace the sensor if any obtained reading does not fall within the specified range or value.

Also, note that continuity and resistance values must match specified values exactly, or very nearly so, but in all cases, obtained readings MUST fall within the ranges specified by the manufacturer. If discrepancies are found, make repairs or replace wiring as required to ensure that all electrical values fall within the manufacturer’s specifications. Clear all codes after repairs are complete, and operate the vehicle for at least one complete drive cycle to see if the code returns.

WARNING: Take note that extreme care MUST be taken when testing currents at any pin on any PCM, since accidental contact with the wrong pin or terminal can cause severe damage to the PCM. Perform these tests in strict accordance with the instructions in the manual, or refer the vehicle for professional diagnosis and repair if you are not comfortable with the idea of testing circuits directly on the PCM.

Step 5

Although Steps 1 through 4 will resolve code P0168 in a large percentage of cases, some stubborn cases may not be resolvable without using an oscilloscope to properly analyze the signal generated by the fuel composition/temperature sensor.

Thus, if an oscilloscope is not available, resist the temptation to throw parts at the problem in the hope of hitting the solution by accident. In these cases, the wisest course of action is always to refer the vehicle to the dealer or other competent repair shop for professional diagnosis and repair.

Codes Related to P0168

P0169 – Relates to “Incorrect Fuel Composition”

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