The DEF level sensor, which is part of the DEF sending unit, measures the level of fluid remaining in the DEF tank. When the liquid level is low, the DEF level sensor sends a signal to the vehicle's ECU which then triggers the warning light on the gauges or panel.

How does a DEF quality sensor work?

Ultrasound waves are used in Diesel Exhaust Fluid (DEF) Quality Sensors, also known as Fluid Property Sensors, to detect the condition of DEF in a truck's tank. ... If DEF has been diluted with any kind of fluid, such as water, the density of the DEF will be changed and the Quality Sensor will detect it and alert you.

What is a DEF head sensor?

Essentially, the DEF head monitors the DEF system. The DEF head communicates the quality (or concentration of urea) of the fluid is within spec. It relates that there is, in fact, DEF in the tank (quantity). This part also reports the temperature of the DEF is in a safe range.