What does it 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, s