The Water in Fuel Sensor or WiF sensor indicates the presence of water in the fuel. It is installed in the fuel filter

Fuel filter

A fuel filter is a filter in the fuel line that screens out dirt and rust particles from the fuel, normally made into cartridges containing a filter paper. They are found in most internal combustion engines....

 and when the water level in the water separator reaches the warning level, the Wif send an electrical signal to the ECU

Engine control unit

An engine control unit is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running...

 or to dashboard (lamp).
The Wif is used especially in the Common Rail

Common rail

Common rail direct fuel injection is a modern variant of direct fuel injection system for petrol and diesel engines.On diesel engines, it features a high-pressure fuel rail feeding individual solenoid valves, as opposed to low-pressure fuel pump feeding unit injectors...

 engines to avoid the unit injector

Unit Injector

Unit Injector is an integrated direct fuel injection system for diesel engines, combining the injector nozzle and the injection pump in a single component...

 damage.

The Wif sensor uses the difference of electric conductivity through water and diesel fuel by 2 electrodes.

First generation Wif sensor uses a potting resin to isolate the electronic circuit, last generation of Wif sensor (the WS3 sensor in Surface-mount technology

Surface-mount technology

Surface mount technology is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards . An electronic device so made is called a surface mount device...

) is made totally without leakage using an innovative co-moulding process.

The last generation of water in fuel sensor has a high resistance to vibrations and to thermal excursion cycles.

The main Automotive WiF designer and producer is SIGMAR.