DTC P0181 OBDII code is set when the PCM (Powertrain Control Module) encounters a voltage signal on the Fuel Temperature Sensor (FTF) circuit that is not within a range determined by the manufacturer.
The Fuel Temperature Sensor (FTF) is usually part of the Fuel Composition Sensor (FCS). It gives the PCM an accurate reading of the fuel composition and temperature. It is located in the line between the fuel tank and the fuel rail.
If the PCM detects an input signal from the FCS Sensor which shows that the fuel temperature is not within the expected range or if the fuel temperature varies by a greater degree than expected at ambient temperature, this code will illuminate along with the Check Engine light.